test/silly: Extend include path.
test/silly: Extend include path.

When attempting to build uClibc under buildroot, including building the
tests, the silly tests don't currently compile, a result of attempting
to build using a compiler that does not yet have an installed version of
uClibc available.  The error is a missing header file, specifically
atomic.h.

Taking inspiration from the nptl tests, I have extended the EXTRA_CFLAGS
variable to add the required include paths.  The tests can now be built
under buildroot.

Signed-off-by: Andrew Burgess <73f46fa54e57e834aa9e13887532977467ddb664@embecosm.com>
Acked-by: Vineet Gupta <af669d95a4c0de051445d4cafdbb2c5c4a2838be@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <ce1ac9e9ad16abccd7821f371ad381197b4768ac@gmail.com>

foss-for-synopsys-dwc-arc-processors/uClibc,wbx-github/uclibc-ng,kraj/uclibc-ng,wbx-github/uclibc-ng,brgl/uclibc-ng,brgl/uclibc-ng,brgl/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,foss-for-synopsys-dwc-arc-processors/uClibc,kraj/uclibc-ng,wbx-github/uclibc-ng,foss-for-synopsys-dwc-arc-processors/uClibc,brgl/uclibc-ng,kraj/uclibc-ng,kraj/uclibc-ng,wbx-github/uclibc-ng