powerpc: Make function btext_initialize static

This function can be static, make it so, this fix a warning treated as
error with W=1:

  arch/powerpc/kernel/btext.c:173:5: error: no previous prototype for ‘btext_initialize’ [-Werror=missing-prototypes]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/btext.c

@@ -170,7 +170,7 @@ void btext_map(void)
 	boot_text_mapped = 1;
 }
 
-int btext_initialize(struct device_node *np)
+static int btext_initialize(struct device_node *np)
 {
 	unsigned int width, height, depth, pitch;
 	unsigned long address = 0;