doc: enable warnings in doxygen

We want to see missed links
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

doc/libinput.doxygen.in

@@ -7,6 +7,7 @@ OPTIMIZE_OUTPUT_FOR_C  = YES
 EXTRACT_ALL            = YES
 EXTRACT_STATIC         = YES
 MAX_INITIALIZER_LINES  = 0
+WARNINGS               = YES
 QUIET                  = YES
 INPUT                  = "@builddir@"
 FILTER_PATTERNS        = *.h *.dox