8323077: C type error (incompatible function pointer) in X11GLContext.c

Submitted upstream: <https://github.com/openjdk/jfx/pull/1319>

diff --git a/modules/javafx.graphics/src/main/native-prism-es2/x11/X11GLContext.c b/modules/javafx.graphics/src/main/native-prism-es2/x11/X11GLContext.c
index f3fdf6107d731a3c..d8a4c676352f0819 100644
--- a/modules/javafx.graphics/src/main/native-prism-es2/x11/X11GLContext.c
+++ b/modules/javafx.graphics/src/main/native-prism-es2/x11/X11GLContext.c
@@ -275,7 +275,8 @@ JNIEXPORT jlong JNICALL Java_com_sun_prism_es2_X11GLContext_nInitialize
                 dlsym(RTLD_DEFAULT, "glXSwapIntervalSGI");
 
         if (ctxInfo->glXSwapIntervalSGI == NULL) {
-            ctxInfo->glXSwapIntervalSGI = glXGetProcAddress("glXSwapIntervalSGI");
+            ctxInfo->glXSwapIntervalSGI = (PFNGLXSWAPINTERVALSGIPROC)
+                glXGetProcAddress((const GLubyte *)"glXSwapIntervalSGI");
         }
 
     }