Fixed pthread.h not included by GLX header.

src/glx_context.c

@@ -30,7 +30,6 @@
 #include <string.h>
 #include <stdlib.h>
 #include <assert.h>
-#include <pthread.h>
 
 
 // This is the only glXGetProcAddress variant not declared by glxext.h

src/glx_platform.h

@@ -41,6 +41,8 @@
  #include <dlfcn.h>
 #endif
 
+#include <pthread.h>
+
 // We support four different ways for getting addresses for GL/GLX
 // extension functions: glXGetProcAddress, glXGetProcAddressARB,
 // glXGetProcAddressEXT, and dlsym