Fix a compile error:

 ...
   CC       util/libunwind/x86_32.o
 In file included from util/libunwind/x86_32.c:33:0:
 util/libunwind/../../arch/x86/util/unwind-libunwind.c: In function 'libunwind__x86_reg_id':
 util/libunwind/../../arch/x86/util/unwind-libunwind.c:110:11: error: 'EINVAL' undeclared (first use in this function)
    return -EINVAL;
            ^
 util/libunwind/../../arch/x86/util/unwind-libunwind.c:110:11: note: each undeclared identifier is reported only once for each function it appears in
 mv: cannot stat 'util/libunwind/.x86_32.o.tmp': No such file or directory
 make[4]: *** [util/libunwind/x86_32.o] Error 1
 make[3]: *** [util] Error 2
 make[2]: *** [libperf-in.o] Error 2
 make[1]: *** [sub-make] Error 2
 make: *** [all] Error 2

It happens when libunwind-x86 feature is detected.
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20171206015040.114574-1-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>