package/pigpio: install to staging

The pigpio package installs programs and libraries to target, but does
not install the libraries and its headers to staging, while they may
be used by other packages. Let's install them, as was requested in bug

Fixes:

  https://bugs.busybox.net/show_bug.cgi?id=11741



Cc: vishalbhalani89@gmail.com
Cc: ivan.nazarenko@gmail.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>