ANDROID: Fix build error with CONFIG_UCLAMP_TASK disabled

After 7b26719a ("ANDROID: GKI: use internal type definitions in
vendor hooks") we stopped providing a forward declaration of 'struct
uclamp_se' and instead relied on pulling its definition from
linux/sched.h. the 'uclamp_se' structure is conditionally defined based
upon CONFIG_UCLAMP_TASK therefore causing a build error to show up for
the android_rvh_uclamp_eff_get trace point.

Fix this by providing a forward declaration of 'struct uclamp_se' like
before.

Fixes: 7b26719a ("ANDROID: GKI: use internal type definitions in vendor hooks")
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Change-Id: Iaa803d149150c1703435f9cfa52ea1406a9521fd