mk: fix link to combined library

mk: fix link to combined library

The application should be linked to the single combined library in the
condition that both of CONFIG_RTE_BUILD_COMBINE_LIB and
CONFIG_RTE_BUILD_SHARED_LIB are enabled.

The current makefile generates an application that links to each library.
This patch fixes to link the single library.

Before
$ ldd x86_64-ivshmem-linuxapp-gcc/app/test
	linux-vdso.so.1 =>  (0x00007fff232a1000)
	librte_distributor.so => not found
	librte_kni.so => not found
	librte_ivshmem.so => not found
	librte_pipeline.so => not found
	librte_table.so => not found
	librte_port.so => not found
	librte_timer.so => not found
	librte_hash.so => not found
	librte_lpm.so => not found
	librte_power.so => not found
	librte_acl.so => not found
	librte_meter.so => not found
	librte_sched.so => not found
	libm.so.6 => /lib64/libm.so.6 (0x00007fc638020000)
	librt.so.1 => /lib64/librt.so.1 (0x00007fc637e18000)
	librte_kvargs.so => not found
	librte_mbuf.so => not found
	librte_ip_frag.so => not found
	libethdev.so => not found
	librte_malloc.so => not found
	librte_mempool.so => not found
	librte_ring.so => not found
	librte_eal.so => not found
	librte_cmdline.so => not found
	librte_cfgfile.so => not found
	librte_pmd_bond.so => not found
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007fc637bfe000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fc6379fa000)
	libintel_dpdk.so => not found
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fc6377dd000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fc63741c000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fc638330000)

After
$ ldd x86_64-ivshmem-linuxapp-gcc/app/test
	linux-vdso.so.1 =>  (0x00007fffb79fe000)
	librt.so.1 => /lib64/librt.so.1 (0x00007f0d8a971000)
	libm.so.6 => /lib64/libm.so.6 (0x00007f0d8a66f000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007f0d8a458000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007f0d8a254000)
	libintel_dpdk.so => not found
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f0d8a037000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f0d89c76000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f0d8ab82000)

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Reviewed-by: Hayato Momma <h-momma@ce.jp.nec.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>