alpha: fix x86.git merge build error

a5a19c63f4e55e32dc0bc3d936d7f94793d8b380 removed the include of
asm/pgalloc.h from asm-generic/tlb.h. That works fine on most
architectures, but broke ALPHA.

Fixup ALPHA by adding the include to asm-alpha/tlbflush.h

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-alpha/tlbflush.h

@@ -3,6 +3,7 @@
 
 #include <linux/mm.h>
 #include <asm/compiler.h>
+#include <asm/pgalloc.h>
 
 #ifndef __EXTERN_INLINE
 #define __EXTERN_INLINE extern inline