[ARM] Fix XIP_KERNEL build error in arch/arm/mm/mmu.c

XIP kernels need to know the start/end of text, but we were
missing the declaration of _etext in mmu.c.  Add it.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/mm/mmu.c

@@ -27,7 +27,7 @@
 
 DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
 
-extern void _stext, __data_start, _end;
+extern void _stext, _etext, __data_start, _end;
 extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
 
 /*