tools/libxl: do not use -c -E compiler options together

It makes no sense to request for preprocessor only output and also request
object file generation.  Fix the _libxl.api-for-check target to only use
-E (preprocessor output).

Also Clang 20.0 reports an error if both options are used.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Fixes: 2862bf5b6c ('libxl: enforce prohibitions of internal callers')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>
(cherry picked from commit a235f856e4)

tools/libs/light/Makefile

@@ -195,7 +195,7 @@ libxl.api-ok: check-libxl-api-rules _libxl.api-for-check
 	touch $@
 
 _libxl.api-for-check: $(XEN_INCLUDE)/libxl.h
-	$(CC) $(CPPFLAGS) $(CFLAGS) -c -E $< $(APPEND_CFLAGS) \
+	$(CC) $(CPPFLAGS) $(CFLAGS) -E $< $(APPEND_CFLAGS) \
 		-DLIBXL_EXTERNAL_CALLERS_ONLY=LIBXL_EXTERNAL_CALLERS_ONLY \
 		>$@.new
 	mv -f $@.new $@