package/pigpio: add soversion to shared libraries

package/pigpio: add soversion to shared libraries

The SONAME of the libraries installed by pigpio now contain a major
version number:

$ readelf -d output/target/usr/lib/libpigpio* | grep soname
 0x0000000e (SONAME)                     Library soname: [libpigpiod_if2.so.1]
 0x0000000e (SONAME)                     Library soname: [libpigpiod_if.so.1]
 0x0000000e (SONAME)                     Library soname: [libpigpio.so.1]

So we must make sure they are installed with this name, and not just
with a .so extension. For consistency, we do the same logic for both
the target and the staging installation.

Signed-off-by: Grzegorz Blach <grzegorz@blach.pl>
[Thomas: improve commit log]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>