mm: Do not forcibly compile HIGHMEM

64-bit systems do not include CONFIG_HIGHMEM because there is a maximum
of 2^64 (or around 16.7 million terabytes) of virtual memory space.
However, even with CONFIG_HIGHMEM disabled, mm compiles highmem.c along
with CONFIG_MMU. Do not compile a useless kernel module if we do not use
it.

Signed-off-by: Tyler Nijmeh <tylernij@gmail.com>
Signed-off-by: Dušan Uverić <dusan.uveric9@gmail.com>
Signed-off-by: ImSpiDy <spidy2713@gmail.com>
Signed-off-by: ImPrashantt <prashant33968@gmail.com>
Signed-off-by: improbir <ghosh_probir@yahoo.com>