krb5: specify krb5_enomem() calling convention

krb5_enomem() was lacking a calling convention decoration, breaking Windows
consumers if __cdecl was not the default calling convention. Add KRB5_LIB_CALL
to make it consistent with other library functions.

Closes: #834

lib/krb5/enomem.c

@@ -33,7 +33,7 @@
 
 #include "krb5_locl.h"
 
-krb5_error_code
+KRB5_LIB_FUNCTION krb5_error_code KRB5_LIB_CALL
 krb5_enomem(krb5_context context)
 {
     krb5_set_error_message(context, ENOMEM, N_("malloc: out of memory", ""));