oid.h: remove MBEDTLS_MD_C guard

*** Comparing before-default -> after-default ***
   x509parse: total 723; skipped  26 ->  26
   x509write: total  41; skipped   8 ->   8
         pem: total  13; skipped   0 ->   0
         oid: total  28; skipped   0 ->   0

*** Comparing before-full -> after-full ***
   x509parse: total 723; skipped  25 ->  25
   x509write: total  41; skipped   0 ->   0
         pem: total  13; skipped   0 ->   0
         oid: total  28; skipped   0 ->   0

*** Comparing reference -> drivers ***
   x509parse: total 723; skipped  89 ->  89
   x509write: total  41; skipped   3 ->   3
         pem: total  13; skipped   0 ->   0
         oid: total  28; skipped   0 ->   0

Signed-off-by: Przemek Stekiel <przemyslaw.stekiel@mobica.com>
diff --git a/include/mbedtls/oid.h b/include/mbedtls/oid.h
index a36a20f..4ee3f93 100644
--- a/include/mbedtls/oid.h
+++ b/include/mbedtls/oid.h
@@ -34,9 +34,7 @@
 #include "mbedtls/cipher.h"
 #endif
 
-#if defined(MBEDTLS_MD_C)
 #include "mbedtls/md.h"
-#endif
 
 /** OID is not found. */
 #define MBEDTLS_ERR_OID_NOT_FOUND                         -0x002E