dma-mapping: fix build errors on !HAS_DMA architectures

commit 4565f01 "dma-mapping: unify
dma_get_cache_alignment implementations" causes build errors on
!HAS_DMA architectures/platforms like s390 and sun3:

include/linux/dma-mapping.h:145: error: static declaration of 'dma_get_cache_alignment' follows non-static declaration
include/asm-generic/dma-mapping-broken.h:73: error: previous declaration of 'dma_get_cache_alignment' was here

Fix this by adding an explicit ifdef.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/dma-mapping.h

@@ -142,13 +142,15 @@ static inline int dma_set_seg_boundary(struct device *dev, unsigned long mask)
 		return -EIO;
 }
 
+#ifdef CONFIG_HAS_DMA
 static inline int dma_get_cache_alignment(void)
 {
 #ifdef ARCH_DMA_MINALIGN
 	return ARCH_DMA_MINALIGN;
 #endif
 	return 1;
 }
+#endif
 
 /* flags for the coherent memory api */
 #define	DMA_MEMORY_MAP			0x01