Fix libgmp for Android

Android terminated immediately after start since commit
181f7baa45
because library wasn't loaded, and libminetest.so required
libgmp.so.10 instead of libgmp.so.

The second issue has been solved by the linker flags.

build/android/src/net/minetest/minetest/MtNativeActivity.java

@@ -86,6 +86,12 @@ public class MtNativeActivity extends NativeActivity {
 		System.loadLibrary("vorbis");
 		System.loadLibrary("ssl");
 		System.loadLibrary("crypto");
+		System.loadLibrary("gmp");
+
+		// We don't have to load libminetest.so ourselves,
+		// but if we do, we get nicer logcat errors when
+		// loading fails.
+		System.loadLibrary("minetest");
 	}
 	
 	private int m_MessagReturnCode;