glx: get rid of memory leak

It fixes Coverity issue: `CID 1468356:  Resource leaks  (RESOURCE_LEAK)`

Fixes: e1964496 ("glx: initial plumbing to let users force-enable/disable extensions")
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7310>
diff --git a/src/glx/glxextensions.c b/src/glx/glxextensions.c
index c049073..10d4faf 100644
--- a/src/glx/glxextensions.c
+++ b/src/glx/glxextensions.c
@@ -502,6 +502,8 @@
                  enable ? "enable" : "disable", field);
       }
    }
+
+   free(env);
 }
 
 /**