Suppress compiler warnings with specific bison and compiler combinations

Bison generates code that only increases the yynerrs counter, it's never
read.  This causes a warning in newer compilers (in particular clang).
Newer versions of bison mark yynerrs with __attribute__((unused)), but
at least on FreeBSD 14 that's not yet available.

src/libstrongswan/settings/settings_parser.y

@@ -193,6 +193,8 @@ value:
 		}
 		free($1);
 		free($2);
+		/* suppress compiler warning */
+		(void)yynerrs;
 	}
 	;
 

src/starter/parser/parser.y

@@ -201,6 +201,8 @@ value:
 		}
 		free($1);
 		free($2);
+		/* suppress compiler warning */
+		(void)yynerrs;
 	}
 	;