fix inclusion of private_key_t in nm plugin

src/charon/plugins/nm/nm_creds.h

@@ -21,6 +21,7 @@
 #ifndef NM_CREDS_H_
 #define NM_CREDS_H_
 
+#include <credentials/keys/private_key.h>
 #include <credentials/credential_set.h>
 
 typedef struct nm_creds_t nm_creds_t;