rrdtool: fix build by disabling docs
rrdtool: fix build by disabling docs

Have no idea what caused it but on all of my build machines started to fail
with recently:

| Attempt to reload Encode.pm aborted.
| Compilation failed in require at /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-mortsgna-linux-gnueabi/rrdtool/1.7.0-r0/recipe-sysroot-native/usr/bin/perl-native/../../../usr/lib/perl5/5.28.1/Pod/Text.pm line 33.
| BEGIN failed--compilation aborted at /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-mortsgna-linux-gnueabi/rrdtool/1.7.0-r0/recipe-sysroot-native/usr/bin/perl-native/../../../usr/lib/perl5/5.28.1/Pod/Text.pm line 33.
| Compilation failed in require at (eval 7) line 1.
| BEGIN failed--compilation aborted at /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-mortsgna-linux-gnueabi/rrdtool/1.7.0-r0/recipe-sysroot-native/usr/bin/perl-native/../../../usr/lib/perl5/5.28.1/Pod/Usage.pm line 30.
| Compilation failed in require at /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-mortsgna-linux-gnueabi/rrdtool/1.7.0-r0/recipe-sysroot-native/usr/bin/pod2man line 19.
| BEGIN failed--compilation aborted at /home/superandy/tmp/oe-core-glibc/work/cortexa7t2hf-neon-vfpv4-mortsgna-linux-gnueabi/rrdtool/1.7.0-r0/recipe-sysroot-native/usr/bin/pod2man line 19.
| make[1]: *** [Makefile:692: rrddump.1] Error 255
| make[1]: *** Waiting for unfinished jobs....

Signed-off-by: Andreas Müller <d72a15aaaca3a7afa855175c72dc8d2e05931e32@gmail.com>
Signed-off-by: Khem Raj <729d64b6f67515e258459a5f6d20ec88b2caf8df@gmail.com>

mrchapp/meta-openembedded,openembedded/meta-openembedded,victronenergy/meta-openembedded,openembedded/meta-openembedded,openembedded/meta-openembedded,mrchapp/meta-openembedded,VCTLabs/meta-openembedded,schnitzeltony/meta-openembedded,victronenergy/meta-openembedded,schnitzeltony/meta-openembedded,rehsack/meta-openembedded,schnitzeltony/meta-openembedded,mrchapp/meta-openembedded,lgirdk/meta-openembedded,rehsack/meta-openembedded,rehsack/meta-openembedded,VCTLabs/meta-openembedded,victronenergy/meta-openembedded,schnitzeltony/meta-openembedded,openembedded/meta-openembedded,mrchapp/meta-openembedded,VCTLabs/meta-openembedded,lgirdk/meta-openembedded,moto-timo/meta-openembedded,lgirdk/meta-openembedded,victronenergy/meta-openembedded,openembedded/meta-openembedded,rehsack/meta-openembedded,schnitzeltony/meta-openembedded,mrchapp/meta-openembedded,moto-timo/meta-openembedded,lgirdk/meta-openembedded,victronenergy/meta-openembedded,victronenergy/meta-openembedded,rehsack/meta-openembedded,openembedded/meta-openembedded,VCTLabs/meta-openembedded,openembedded/meta-openembedded,victronenergy/meta-openembedded,VCTLabs/meta-openembedded,rehsack/meta-openembedded,lgirdk/meta-openembedded,schnitzeltony/meta-openembedded,moto-timo/meta-openembedded,VCTLabs/meta-openembedded,lgirdk/meta-openembedded,moto-timo/meta-openembedded,mrchapp/meta-openembedded,VCTLabs/meta-openembedded,VCTLabs/meta-openembedded,schnitzeltony/meta-openembedded,mrchapp/meta-openembedded,openembedded/meta-openembedded,lgirdk/meta-openembedded,moto-timo/meta-openembedded,rehsack/meta-openembedded