image: Add an inline declaration of unmap_sysmem()

Add an empty inline declaration when compiling tools for a host where
unmap_sysmem() is not defined.

Signed-off-by: Paul HENRYS <paul.henrys_ext@softathome.com>

tools/mkimage.h

@@ -37,6 +37,10 @@ static inline void *map_sysmem(ulong paddr, unsigned long len)
 	return (void *)(uintptr_t)paddr;
 }
 
+static inline void unmap_sysmem(const void *vaddr)
+{
+}
+
 static inline ulong map_to_sysmem(const void *ptr)
 {
 	return (ulong)(uintptr_t)ptr;