build: Enable -Wdiscarded-qualifiers

The codebase is clean for it, so why not enforce it?

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://phabricator.endlessm.com/T24004

meson.build

@@ -55,6 +55,7 @@ test_c_args = [
   '-Wcast-align',
   '-Wclobbered',
   '-Wno-declaration-after-statement',
+  '-Wdiscarded-qualifiers',
   '-Wduplicated-branches',
   '-Wduplicated-cond',
   '-Wempty-body',
@@ -78,7 +79,6 @@ test_c_args = [
   '-Wmissing-prototypes',
   '-Wnested-externs',
   '-Wno-error=cpp',
-  '-Wno-discarded-qualifiers',
   '-Wno-missing-field-initializers',
   '-Wno-suggest-attribute=format',
   '-Wno-unused-parameter',