Clang can now find _u32_getbits symbol at link time

Apple clang version 4.0 fails with:
```
cc -I../inc -o tart tart.o ../lib/libtart.a
Undefined symbols for architecture x86_64:
  "_u32_getbits", referenced from:
      _test_getbits in libtart.a(number.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [tart] Error 1
```
Declaring `u32_getbits(...)` in `number.h` resolves the issue.