perfc: fix build after commit fc32575968 when CONFIG_PERF_COUNTERS=y

perfc: fix build after commit fc32575968 when CONFIG_PERF_COUNTERS=y

The commit fc32575968 "public/sysctl: drop unnecessary typedefs and
handles" went a bit too far by replacing all xen_systcl_*_t type to
struct xen_sysctl_*.

However, xen_sysctl_perfc_val_t was a typedef on uint32_t and therefore
is not associated to a structure.

Use xen_sysctl_perfc_val_t to fix the build when CONFIG_PERF_COUNTERS=y

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>