perf: use libunwind

Without libunwind perf does not show userspace stack frames.
Tested on mvebu.

Signed-off-by: Maxim Gorbachyov <maxim.gorbachyov@gmail.com>

package/devel/perf/Makefile

@@ -26,7 +26,7 @@ include $(INCLUDE_DIR)/package.mk
 define Package/perf
   SECTION:=devel
   CATEGORY:=Development
-  DEPENDS:= +libelf1 +libdw +libpthread +librt +objdump @!LINUX_3_18 @!IN_SDK @!TARGET_arc770 @KERNEL_PERF_EVENTS
+  DEPENDS:= +libelf1 +libdw +libunwind +libpthread +librt +objdump @!LINUX_3_18 @!IN_SDK @!TARGET_arc770 @KERNEL_PERF_EVENTS
   TITLE:=Linux performance monitoring tool
   VERSION:=$(LINUX_VERSION)-$(PKG_RELEASE)
   URL:=http://www.kernel.org