boot/bootutil: Add missing #include

Mynewt builds require the `mcuboot_config.h` file to translate syscfg
setting names to MCUboot setting names.  This change fixes a build error
that occurs when MCUboot is built with mbedTLS support:

Error: In file included from keys/bootkeys/src/bootkeys.c:2:0:
repos/mcuboot/boot/bootutil/include/bootutil/enc_key.h:29:10: fatal error: tinycrypt/aes.h: No such file or directory
 #include "tinycrypt/aes.h"
          ^~~~~~~~~~~~~~~~~

Signed-off-by: Christopher Collins <ccollins@apache.org>
diff --git a/boot/bootutil/include/bootutil/enc_key.h b/boot/bootutil/include/bootutil/enc_key.h
index 47ec6c2..d2477ca 100644
--- a/boot/bootutil/include/bootutil/enc_key.h
+++ b/boot/bootutil/include/bootutil/enc_key.h
@@ -21,6 +21,7 @@
 #define BOOTUTIL_ENC_KEY_H
 
 #include <flash_map_backend/flash_map_backend.h>
+#include "mcuboot_config/mcuboot_config.h"
 #include "bootutil/image.h"
 
 #if defined(MCUBOOT_USE_MBED_TLS)