Makefile: explictly link with all shared libraries

linking with libuci means we need to link with libblobmsg_json, libjson-c and
libubox. Sinc e we do not use pkg-config or something equivalent, we need to
specify all dependencies.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

Makefile

 IWINFO_BACKENDS    = $(BACKENDS)
 IWINFO_CFLAGS      = $(CFLAGS) -std=gnu99 -fstrict-aliasing -Iinclude
-IWINFO_LDFLAGS     = -luci
+IWINFO_LDFLAGS     = -luci -lblobmsg_json -ljson-c -lubox
 
 IWINFO_LIB         = libiwinfo.so
 IWINFO_LIB_LDFLAGS = $(LDFLAGS) -shared