INSTALL.md: Add note about EXTRA_CFLAGS.

INSTALL.md: Add note about EXTRA_CFLAGS.

Add a note about the use of EXTRA_CFLAGS to provide custom CFLAGS for
the build of the Linux kernel module.

This addition is technically in the "configuring the sources" section
of the document.  However, it's the spot where custom CFLAGS is
discussed already, so that seemed like the best place to put it.
Alternatively, it could go in the "Building the Sources" section
instead.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>