Input: cyttsp - include appropriate header file in cyttsp_i2c_common.c

Include cyttsp4_core.h in cyttsp_i2c_common.c to ensure that implementation
of cyttsp_i2c_read_block_data() and cyttsp_i2c_write_block_data()
match what the rest of the driver expects.

Thus, it also eliminates the following warning in cyttsp_i2c_common.c:
drivers/input/touchscreen/cyttsp_i2c_common.c:34:5: warning: no previous prototype for ‘cyttsp_i2c_read_block_data’ [-Wmissing-prototypes]
drivers/input/touchscreen/cyttsp_i2c_common.c:64:5: warning: no previous prototype for ‘cyttsp_i2c_write_block_data’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>