powerpc64/ftrace: Include ftrace.h needed for enable/disable calls

this_cpu_disable_ftrace and this_cpu_enable_ftrace are inlines in
ftrace.h Without it included, the build fails.

Fixes: a4bc64d3 ("powerpc64/ftrace: Disable ftrace during kvm entry/exit")
Cc: stable@vger.kernel.org # v4.18+
Signed-off-by: Luke Dashjr <luke-jr+git@utopios.org>
Acked-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kvm/book3s_hv.c

@@ -46,6 +46,7 @@
 #include <linux/compiler.h>
 #include <linux/of.h>
 
+#include <asm/ftrace.h>
 #include <asm/reg.h>
 #include <asm/ppc-opcode.h>
 #include <asm/asm-prototypes.h>