ARM: 9306/1: cacheflush: avoid __flush_anon_page() missing-prototype warning

The prototype for __flush_anon_page() is intentionally hidden
inside the flush_anon_page() inline function to prevent it from
being called from drivers.

When building with 'W=1', this causes a warning:

arch/arm/mm/flush.c:358:6: error: no previous prototype for '__flush_anon_page' [-Werror=missing-prototypes]

Work around this by adding a prototype directly next to the function
definition.

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>