core/logging: Fix broken syslog levels on older glibc.

The fix to ASTERISK-25407 introduced the usage of LOG_MAKEPRI. However
this macro is broken in older glibc (< 2.17); it would left-shift the
facility a second time, causing the resultant priority to become
invalid.

The syslog manpage mentions nothing about LOG_MAKEPRI and suggests this:

    The priority argument is formed by ORing the facility and the level
    values [...].

ASTERISK-25510 #close
Reported by: Michael Newton

Change-Id: Ia89debe7fac5ad090c7ef595c0707f31bb1e3d03