x86: highmem: Restore the not so leftover function prototype

commit e31b7991 (x86: highmem: Remove leftover function prototypes)
removed kmap_atomic_prot_pfn() which is not a leftover, but should
have been left where it was.

Restore it.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/include/asm/highmem.h

@@ -69,6 +69,8 @@ void __kunmap_atomic(void *kvaddr, enum km_type type);
 void *__kmap_atomic_pfn(unsigned long pfn, enum km_type type);
 struct page *__kmap_atomic_to_page(void *ptr);
 
+void *kmap_atomic_prot_pfn(unsigned long pfn, enum km_type type, pgprot_t prot);
+
 #ifndef CONFIG_PARAVIRT
 #define kmap_atomic_pte(page, type)		kmap_atomic(page, type)
 #define kmap_atomic_pte_direct(page, type)	kmap_atomic_direct(page, type)