doc-rst: parse-headers: add an option to ignore enum symbols

At videodev2.h, we have hundreds of symbols that don't
currently have a reference yet. Let's ignore for how, while
we don't improve those cross-refs.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/sphinx/parse-headers.pl

@@ -163,6 +163,10 @@ if ($file_exceptions) {
 			delete $structs{$1} if (exists($structs{$1}));
 			next;
 		}
+		if (m/^ignore\s+symbol\s+(\S+)/) {
+			delete $enum_symbols{$1} if (exists($enum_symbols{$1}));
+			next;
+		}
 
 		# Parsers to replace a symbol