Fix compiler warnings

[argilo]

There are some compiler warnings that should be fixed:

In file included from /home/argilo/git/gr-dsd-build/include/dsd_block_ff.h:30:0,
from /home/argilo/git/gr-dsd-build/lib/dsd_block_ff.cc:32:
/home/argilo/git/gr-dsd-build/dsd/dsd.h:23:0: warning: "__USE_XOPEN" redefined [enabled by default]
/usr/include/features.h:251:0: note: this is the location of the previous definition

/usr/bin/ld: Warning: alignment 4 of symbol exitflag' in CMakeFiles/gr-dsd.dir/dsd_block_ff.cc.o is smaller than 16 in ../dsd/libdsd.a(dsd_mbe.c.o) /usr/bin/ld: Warning: alignment 4 of symbolexitflag' in CMakeFiles/gr-dsd.dir/dsd_block_ff.cc.o is smaller than 16 in ../dsd/libdsd.a(dsd_frame_sync.c.o)

In file included from /home/argilo/git/gr-dsd-build/include/dsd_block_ff.h:30:0,
from /home/argilo/git/gr-dsd-build/swig/dsd_swigPYTHON_wrap.cxx:4370:
/home/argilo/git/gr-dsd-build/dsd/dsd.h:23:0: warning: "__USE_XOPEN" redefined [enabled by default]
/usr/include/features.h:251:0: note: this is the location of the previous definition