mm2gv: squash -Wconversion warnings for 'tolower' usage

mm2gv: squash -Wconversion warnings for 'tolower' usage

For historical reasons, C functions/macros like `tolower` are specified to take
and return an `int`. This means sometimes one set of pedantic compilers (MinGW
with -Wconversion) complains about passing `char` values into these ctype.h
interfaces and another set of pedantic compilers (GCC with -Wconversion)
complains about coercing the result into a `char`. This change suppresses both
of these false positives.