staging: rtl8188eu: include missing header

This patch fixes the following sparse warnings:

drivers/staging/rtl8188eu/hal/phy.c:46:5: warning: symbol
'phy_query_bb_reg' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:56:6: warning: symbol
'phy_set_bb_reg' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:129:5: warning: symbol
'phy_query_rf_reg' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:140:6: warning: symbol
'phy_set_rf_reg' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:228:6: warning: symbol
'phy_set_tx_power_level' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:324:6: warning: symbol
'phy_set_bw_mode' was not declared. Should it be static?
drivers/staging/rtl8188eu/hal/phy.c:360:6: warning: symbol 'phy_sw_chnl'
was not declared. Should it be static?

Signed-off-by: Nicolas Thery <nthery@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>