rsa: fix: use #ifdef MCU_BOOT_RSA_PKCS1_15 instead of #if
As this flag is tested with #ifdef everywhere else, use #ifdef in
image_rsa.c
#include "bootutil_priv.h"
-#if MCUBOOT_RSA_PKCS1_15
+#ifdef MCUBOOT_RSA_PKCS1_15
static const uint8_t sha256_oid[] = {
0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
