Makefile.toolchain: Add TEST_FUZZ checks.
Makefile.toolchain: Add TEST_FUZZ checks.

This prevents a bunch of warnings that show when trying to run
pkg-config for libprotobuf-mutator when not building the fuzzer
targets.

BRANCH=None
BUG=None
TEST=make -j buildall

Change-Id: Idf8de959d86db744754cd237796ccaacd3668a63
Signed-off-by: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1338605
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec