x86: get_cpu_flags: add necessary ifdefs around function body

ff_get_cpu_flags_x86() requires cpuid(), which is conditionally defined
elsewhere in the file.  Surrounding the function body with ifdefs allows
building even when cpuid is not defined.  An empty cpuflags mask is
returned in this case.

libavutil/x86/cpu.c

@@ -89,6 +89,9 @@ static int cpuid_test(void)
 int ff_get_cpu_flags_x86(void)
 {
     int rval = 0;
+
+#ifdef cpuid
+
     int eax, ebx, ecx, edx;
     int max_std_level, max_ext_level, std_caps = 0, ext_caps = 0;
     int family = 0, model = 0;
@@ -192,5 +195,7 @@ int ff_get_cpu_flags_x86(void)
             rval |= AV_CPU_FLAG_ATOM;
     }
 
+#endif /* cpuid */
+
     return rval;
 }