devel/libbacktrace: pass -fPIC to CFLAGS (+)

This need to unbreak build Boost and consumers as relocable objects

ld: error: relocation R_X86_64_32 cannot be used against local symbol; recompile with -fPIC

defined in /usr/local/lib/libbacktrace.a(posix.o)
referenced by posix.o:(backtrace_close) in archive /usr/local/lib/libbacktrace.a

Approved by: portmgr blanket (trivial fix build)