md5: declare byteReverse as static

md5: declare byteReverse as static

byteReverse() is an internal function in md5.c, and is not exposed via
any header file, but it is not declared as static. This is a problem
with the md5.c file since it is copied more or less verbatim in other
programs (fontconfig and pjsip among others), causing a link error
when linking two of them together.

Fixes link failures such as:
  http://autobuild.buildroot.net/results/419ab2c0e034cc68991281c51caa8271b0fadbab/build-end.log

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>