valgrind supps: ignore leak in XextCreateExtension

Searching for that specific function in other suppression files
on the Internet shows a few projects ignoring it

eg: https://github.com/XCSoar/XCSoar/blob/master/valgrind.suppressions

tests/check/gst-plugins-base.supp

@@ -559,3 +559,13 @@
    fun:call_init.part.0
    fun:_dl_init
 }
+{
+   <a one time alloc>
+   Memcheck:Leak
+   match-leak-kinds: definite
+   fun:malloc
+   fun:XextCreateExtension
+   ...
+   fun:glXChooseFBConfig
+   fun:gst_gl_context_glx_choose_format
+}