powerpc/selftest: fix type of mftb() in null_syscall

All callers of mftb() expect 'unsigned long', and the function itself
only returns lower part of the TB so it really is 'unsigned long'
not 'unsigned long long'

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/benchmarks/null_syscall.c

@@ -25,7 +25,7 @@ unsigned long long clock_frequency;
 unsigned long long timebase_frequency;
 double timebase_multiplier;
 
-static inline unsigned long long mftb(void)
+static inline unsigned long mftb(void)
 {
 	unsigned long low;