Boot: Fix error in preprocessor macro

Fix for error in preprocessor macro causing a compile error when
RAM_LOADING is enabled.

Change-Id: Ia25c0a7a53662292cfe480ee3c2625ec52844453
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/bl2/ext/mcuboot/bootutil/src/loader.c b/bl2/ext/mcuboot/bootutil/src/loader.c
index 2b6f655..2807d0b 100644
--- a/bl2/ext/mcuboot/bootutil/src/loader.c
+++ b/bl2/ext/mcuboot/bootutil/src/loader.c
@@ -186,7 +186,7 @@
         return BOOT_EBADIMAGE;
     }
 
-#if MCUBOOT_RAM_LOADING
+#ifdef MCUBOOT_RAM_LOADING
     if (!(hdr->ih_flags & IMAGE_F_RAM_LOAD)) {
         return BOOT_EBADIMAGE;
     }