flowtop: take PKG_CONFIG into account for libnetfilter_conntrack

flowtop: take PKG_CONFIG into account for libnetfilter_conntrack

Use $PKG_CONFIG to determine the linker flags for libnetfilter_conntrack. This
fixes static link failure like the following:

  LD	flowtop
.../usr/x86_64-buildroot-linux-musl/sysroot/usr/lib/../lib64/libnetfilter_conntrack.a(main.o): In function `nfct_open_nfnl':
main.c:(.text+0x52): undefined reference to `nfnl_subsys_open'
main.c:(.text+0x69): undefined reference to `nfnl_subsys_close'
main.c:(.text+0x87): undefined reference to `nfnl_subsys_open'
main.c:(.text+0xa3): undefined reference to `nfnl_subsys_close'
.../usr/x86_64-buildroot-linux-musl/sysroot/usr/lib/../lib64/libnetfilter_conntrack.a(main.o): In function `nfct_open':
main.c:(.text+0xc9): undefined reference to `nfnl_open'
main.c:(.text+0xf0): undefined reference to `nfnl_close'
...

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>