meson: add -Wmissing-prototypes to the build

Meson's warning level maps to -Wall, -Wextra and -Wpedantic.
-Wmissing-prototypes is added by neither of those flag. Consequently,
it is manually added to the build command line arguments.

Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>

meson.build

@@ -58,6 +58,7 @@ cc = meson.get_compiler('c')
 
 global_args = []
 global_args_maybe = [
+	'-Wmissing-prototypes',
 	'-Wno-unused-parameter',
 	'-Wno-shift-negative-value', # required due to Pixman
 	'-Wno-missing-field-initializers',