drivers: platform: Include appropriate header file in mxm-wmi.c

This patch includes appropriate header file linux/mxm-wmi.h in
x86/mxm-wmi.c because functions mxm_wmi_call_mxds(), mxm_wmi_call_mxmx()
and mxm_wmi_supported() have their prototype declaration in
linux/mxm-wmi.h.

Thus, it also eliminates the following warnings in x86/mxm-wmi.c:
drivers/platform/x86/mxm-wmi.c:43:5: warning: no previous prototype for ‘mxm_wmi_call_mxds’ [-Wmissing-prototypes]
drivers/platform/x86/mxm-wmi.c:68:5: warning: no previous prototype for ‘mxm_wmi_call_mxmx’ [-Wmissing-prototypes]
drivers/platform/x86/mxm-wmi.c:93:6: warning: no previous prototype for ‘mxm_wmi_supported’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>