xtensa: enable HAVE_VIRT_CPU_ACCOUNTING_GEN

There's no direct cputime_t manipulation in the xtensa arch code, so
generic virt CPU accounting may be enabled.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Documentation/features/time/virt-cpuacct/arch-support.txt

@@ -27,5 +27,5 @@
     |       sparc: |  ok  |
     |          um: | TODO |
     |         x86: |  ok  |
-    |      xtensa: | TODO |
+    |      xtensa: |  ok  |
     -----------------------

arch/xtensa/Kconfig

@@ -43,6 +43,7 @@ config XTENSA
 	select HAVE_PERF_EVENTS
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
+	select HAVE_VIRT_CPU_ACCOUNTING_GEN
 	select IRQ_DOMAIN
 	select MODULES_USE_ELF_RELA
 	select PERF_USE_VMALLOC