make: fix installation of generated header files

The PROJ_SRC_DIR != PROJ_OBJ_DIR path was missing the directory
creation logic that was in the path for non-generated headers.
PR11903.

(The oversight was copied and pasted from LLVM's Makefile.rules,
where it apparently existed since time immemorial til it was
corrected in r127325.)

llvm-svn: 149551