Input: cyttsp4 - move 'cyttsp4_tch_abs_string' to the only file that references it

Fixes the following W=1 kernel build warning(s):

In file included from drivers/input/touchscreen/cyttsp_i2c_common.c:24:
 drivers/input/touchscreen/cyttsp4_core.h:236:27: warning: ‘cyttsp4_tch_abs_string’ defined but not used [-Wunused-const-variable=]
 236 | static const char  const cyttsp4_tch_abs_string[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~
 In file included from drivers/input/touchscreen/cyttsp4_i2c.c:17:
 drivers/input/touchscreen/cyttsp4_core.h:236:27: warning: ‘cyttsp4_tch_abs_string’ defined but not used [-Wunused-const-variable=]
 236 | static const char * const cyttsp4_tch_abs_string[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~
 In file included from drivers/input/touchscreen/cyttsp4_spi.c:17:
 drivers/input/touchscreen/cyttsp4_core.h:236:27: warning: ‘cyttsp4_tch_abs_string’ defined but not used [-Wunused-const-variable=]
 236 | static const char * const cyttsp4_tch_abs_string[] = {
 | ^~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104162427.2984742-16-lee.jones@linaro.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>