libpcap: fix dependency of install-shared-so make target

libpcap: fix dependency of install-shared-so make target

There seems to be a situation in which a rebuild of libpcap.so is triggered
in the install step of the libpcap Makefile. libpcap.so is the wrong
target, leading to the build failure reported in [1].

Fix the dependency of install-shared-so to $(SHAREDLIB) so the build can
succeed in this case.

[1] https://dev.openwrt.org/ticket/19894

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>