Fix an error when compiling on OS X 10.9.3

Added declaration for maximum and minimum to avoid the 'Undefined symbols...' error when building.

gcc -bundle -o levenshtein.so levenshtein.c -I/usr/local/mysql/include
Undefined symbols for architecture x86_64:
  "_maximum", referenced from:
      _levenshtein_ratio in levenshtein-9d0d03.o
      _levenshtein_k_ratio in levenshtein-9d0d03.o
  "_minimum", referenced from:
      _levenshtein in levenshtein-9d0d03.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)