ODR warning for "main"

"main" is redeclared with a different type in maint.c.  I think this
might have come from my first gdb patch, many many years ago.  While I
wonder if this profiling code is actually useful at all any more, in
the meantime it's simple to fix the declaration.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=22395