Don't allow compilation if endianness isn't known

PIPE_ARCH_UNKNOWN_ENDIAN is used no where else. All #else branches of
ifdef PIPE_ARCH_LITTLE assume big-endian. Not #error'ing out here
only serves to allow bad things to happen.

Signed-off-by: Matt Turner <mattst88@gmail.com>

src/gallium/include/pipe/p_config.h

@@ -127,7 +127,7 @@
 #elif defined(PIPE_ARCH_PPC) || defined(PIPE_ARCH_PPC_64)
 #define PIPE_ARCH_BIG_ENDIAN
 #else
-#define PIPE_ARCH_UNKNOWN_ENDIAN
+#error Unknown Endianness
 #endif
 
 #endif