mate-sensors-applet: fix (some) builds

Expect basic headers to be available universally. The configure script
doesn't actually check for or define HAVE_LOCALE_H, which can cause
breakages when locale.h isn't pulled in through other means already.
Simplify the source file in question, rather than making configure more
complicated. (There are other source files in this package that
unconditionally expect stdio.h to be present, which seems reasonable.)

(gutteridge)