ffprobe: use quotation marks in the xml header output

xmllint (silently) replaces the ' with " when fixing and validating the output
of ffprobe in fate-ffprobe_xsd.

Reviewed-by: Tobias Rapp <t.rapp@noa-archive.com>
Signed-off-by: James Almer <jamrial@gmail.com>

fftools/ffprobe.c

@@ -1682,9 +1682,9 @@ static void xml_print_section_header(WriterContext *wctx)
         wctx->section[wctx->level-1] : NULL;
 
     if (wctx->level == 0) {
-        const char *qual = " xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' "
-            "xmlns:ffprobe='http://www.ffmpeg.org/schema/ffprobe' "
-            "xsi:schemaLocation='http://www.ffmpeg.org/schema/ffprobe ffprobe.xsd'";
+        const char *qual = " xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" "
+            "xmlns:ffprobe=\"http://www.ffmpeg.org/schema/ffprobe\" "
+            "xsi:schemaLocation=\"http://www.ffmpeg.org/schema/ffprobe ffprobe.xsd\"";
 
         printf("<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n");
         printf("<%sffprobe%s>\n",