Fix undefined symbol: clock_gettime

[SM] Extension dbi.mysql.ext.so failed to load: /home/csgo/csgo/addons/sourcemod/extensions/dbi.mysql.ext.so: undefined symbol: clock_gettime

extensions/mysql/AMBuilder

@@ -13,7 +13,8 @@ if SM.mysql_root:
       os.path.join(SM.mysql_root, 'lib', 'libmysqlclient_r.a'),
       '-lz',
       '-lpthread',
-      '-lm'
+      '-lm',
+      '-lrt'
     ]
   elif builder.target_platform == 'windows':
     binary.compiler.postlink += [