add -fPIC to the static lib build

Before this change, if you try to create a shared object which depends on libebur128.a, you would get something like this:

/usr/bin/ld: /home/andy/dev/libgroove/deps/ebur128/build/libebur128.a(ebur128.c.o): relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
/home/andy/dev/libgroove/deps/ebur128/build/libebur128.a: could not read symbols: Bad value

After this change it works as expected.