bsd.gecko.mk: PowerPC no longer needs the __STDC_CONSTANT_MACROS in CFLAGS

sys/cdefs.h after base r227475 always defines STDC_CONSTANT_MACROS
for C++11 while Firefox enforces C++11 since 25.0 and also defines
STDC_CONSTANT_MACROS via mozilla-config.h since 26.0.

PR: 201294
Submitted by: jhibbits
MFH: 2015Q3 (trivial, not on pkg-fallout)