ARM: 9301/1: dma-mapping: hide unused dma_contiguous_early_fixup function

The dma_contiguous_early_fixup() function is a global __weak stub
with an arm specific override, but the declaration is in an #ifdef.

If CONFIG_DMA_CMA is disabled, there is no caller and no prototype,
which adds a warning for the definition:

arm/mm/dma-mapping.c:261:13: error: no previous prototype for 'dma_contiguous_early_fixup' [-Werror=missing-prototypes]

Enclose the definition in the same #ifdef as the prototype to avoid
that and save a few bytes of .init.text.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>