triggerhappy: use target pkg-config

triggerhappy: use target pkg-config

triggerhappy uses pkg-config to detect the systemd library. Make sure it
uses the target pkg-config, not the host one.

Fixes build failure when the host has systemd pkg-config files:

.../host/bin/arm-linux-gcc -static  th-cmd.o cmdsocket.o  -lsystemd -o th-cmd
.../host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/6.4.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: cannot find -lsystemd

Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>