x86/pmu: Fix compilation on 32-bit hosts

When building for 32-bit hosts, the compiler currently complains:

 x86/pmu.c: In function 'check_gp_counters_write_width':
 x86/pmu.c:490:30: error: left shift count >= width of type

Use the correct suffix to avoid this problem.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200616105940.2907-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>