sys: newlib: conditionally ignore wchar warning

For certain versions of newlib this warning is ignored.

sys/newlib/Makefile.include

@@ -6,6 +6,7 @@ ifneq (,$(filter newlib_nano,$(USEMODULE)))
     USE_NEWLIB_NANO = 1
     ifeq ($(shell echo "int main(){} void _exit(int n) {(void)n;while(1);}" | LC_ALL=C $(CC) -xc - -o /dev/null -lc -specs=nano.specs -Wall -Wextra -pedantic 2>&1 | grep -q "use of wchar_t values across objects may fail" ; echo $$?),0)
         CFLAGS += -fshort-wchar
+        LINKFLAGS += -Wl,--no-wchar-size-warning
     endif
   endif
 endif