build: suppress "-Wno-unused-but-set-variable" when building grammar

bison/flex generates rather ugly C code. Need to suppress this warning.

    lib/route/cls/ematch_syntax.c:1368:9: error: variable 'ematch_nerrs' set but not used [-Werror,-Wunused-but-set-variable]
     1368 |     int yynerrs = 0;
          |         ^
    lib/route/cls/ematch_syntax.c:72:25: note: expanded from macro 'yynerrs'
       72 | #define yynerrs         ematch_nerrs
          |                         ^
    1 error generated.

Had one job.

Makefile.am

@@ -411,6 +411,7 @@ nodist_lib_route_libnl_grammar_la_SOURCES = \
 lib_route_libnl_grammar_la_CPPFLAGS = \
 	$(lib_cppflags) \
 	-I$(srcdir)/lib/route \
+	-Wno-unused-but-set-variable \
 	$(NULL)
 
 $(lib_route_libnl_grammar_la_OBJECTS): $(grammar_files_headers)