krb5_enomem to separate source file

krb5_enomem() is a wrapper around krb5_set_error_message() which
is used throughout the lib/krb5 sources.  Some of the lib/krb5
sources are imported into third party projects and those projects
must be able to pull in krb5_enomem() without other baggage.
Create a new source file lib/krb5/enomem.c.

Change-Id: Id109386d48e3e2988b705b82525adf4f1fa5ea98

lib/krb5/NTMakefile

@@ -72,6 +72,7 @@ libkrb5_OBJS =			\
 	$(OBJ)\digest.obj		    \
 	$(OBJ)\dll.obj			    \
 	$(OBJ)\eai_to_heim_errno.obj	    \
+        $(OBJ)\enomem.obj                   \
 	$(OBJ)\error_string.obj		    \
 	$(OBJ)\expand_hostname.obj	    \
 	$(OBJ)\expand_path.obj	    	    \
@@ -216,6 +217,7 @@ dist_libkrb5_la_SOURCES =			\
 	deprecated.c				\
 	digest.c				\
 	eai_to_heim_errno.c			\
+        enomem.c                                \
 	error_string.c				\
 	expand_hostname.c			\
 	expand_path.c				\