Fix building srp_examples.c with musl

This fixes the immediate build failure, but it's quite a fragile fix, because if any other libc defines malloc, calloc, or free in a header that has not been included by this point in srp_examples.c, but is subsequently included by srp.c or srp_constants.c, the same problem will occur. The ideal thing to do would be to find a way to accomplish the desired effect without needing to use a macro to override the definitions of libc functions.