nodeinfo: fix build on non-Linux

SYSFS_SYSTEM_PATH is only defined for Linux, however it's used outside
of #ifdef __linux__ code, e.g. as the first argument to
nodeCapsInitNUMAFake().

But as this argument's value is used on Linux only, it's safe to define
SYSFS_SYSTEM_PATH to "fake" to get things built on FreeBSD.

src/nodeinfo.c

@@ -129,6 +129,8 @@ virNodeGetSiblingsListLinux(const char *dir, int cpu_id)
     VIR_FREE(path);
     return ret;
 }
+#else
+# define SYSFS_SYSTEM_PATH "fake"
 #endif