mut_exec_symtab.c: fix strcmp warning
mut_exec_symtab.c: fix strcmp warning

Compiling under gcc version 6.2.0 20161005 (Ubuntu 6.2.0-5ubuntu12)
(correctly) gives a warning about strcmp not being declared, so
pull in <string.h>.

nineties-retro/mut,nineties-retro/mut,nineties-retro/mut