Ephysics: Silence more warnings from libbullet

The bullet library generates tons of warnings:
1. -Wunused-variable
2. -Wunused-parameter
3. -Wshadow

2 and 3 were properly ignored by pragma GCC, but unused-variable
doesn't seem to work. At least on my version of GCC (4.8.2).
So, let's ignore ALL warnings from libbullet includes.

src/lib/ephysics/ephysics_private.h

@@ -6,8 +6,7 @@
 #endif
 
 #pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunused-parameter"
-#pragma GCC diagnostic ignored "-Wshadow"
+#pragma GCC system_header
 
 #include <BulletSoftBody/btSoftBodyRigidBodyCollisionConfiguration.h>
 #include <BulletSoftBody/btDefaultSoftBodySolver.h>