wimax: export linux/wimax.h and linux/wimax/i2400m.h with headers_install

These two files are what user space can use to establish communication
with the WiMAX kernel API and to speak the Intel 2400m Wireless WiMAX
connection's control protocol.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>