mbedtls: enable MBEDTLS_DHM_C

This option is required by OpenVPN, and OpenVPN 2.4 uses mbedTLS 2.x.
DHM_C is also already enabled in the PolarSSL 1.3.x config.h.

Signed-off-by: Magnus Kroken <mkroken@gmail.com>

package/libs/mbedtls/patches/200-config.patch

@@ -179,15 +179,6 @@
  
  /**
   * \def MBEDTLS_DES_C
-@@ -1740,7 +1740,7 @@
-  * This module is used by the following key exchanges:
-  *      DHE-RSA, DHE-PSK
-  */
--#define MBEDTLS_DHM_C
-+//#define MBEDTLS_DHM_C
- 
- /**
-  * \def MBEDTLS_ECDH_C
 @@ -2158,7 +2158,7 @@
   * Caller:  library/mbedtls_md.c
   *