Fix link error when running `make tools` with skia_shared_lib=1

Fix link error when running `make tools` with skia_shared_lib=1

I'm getting this error, which is fixed by this change:

[75/76] LINK skpdiff
FAILED: c++ -m64 -pie -Wl,-rpath=\$ORIGIN/lib/ -Wl,-rpath-link=lib/ -o skpdiff -Wl,--start-group obj/tools/skpdiff/skpdiff.skpdiff_main.o obj/tools/skpdiff/skpdiff.SkDiffContext.o obj/tools/skpdiff/skpdiff.SkImageDiffer.o obj/tools/skpdiff/skpdiff.SkPMetric.o obj/tools/skpdiff/skpdiff.skpdiff_util.o obj/tools/skpdiff/skpdiff.SkDifferentPixelsMetric_cpu.o obj/gyp/libflags.a obj/gyp/libpicture_utils.a -Wl,--end-group lib/libskia.so -lrt -lz
/usr/bin/ld: obj/tools/skpdiff/skpdiff.SkDiffContext.o: undefined reference to symbol 'pthread_mutexattr_settype@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

BUG=skia:

Review URL: https://codereview.chromium.org/1206333002