meson: reuse libsystemd_static for libsystemd

meson: reuse libsystemd_static for libsystemd

This gets rid of recompilation, making things faster and avoids duplicated warnings.
The result seems to be the same:
$ ls -l build/libsystemd.so.0.20.0 build2/libsystemd.so.0.20.0
-rwxrwxr-x. 1 zbyszek zbyszek 3071312 Dec 19 11:45 build2/libsystemd.so.0.20.0
-rwxrwxr-x. 1 zbyszek zbyszek 3071760 Dec 19 11:11 build/libsystemd.so.0.20.0
$ diff -U1 <(objdump -T build/libsystemd.so.0.20.0|sed -r 's/[0-9a-f]{16}/________________/g') <(objdump -T build2/libsystemd.so.0.20.0|sed -r 's/[0-9a-f]{16}/________________/g')
-build/libsystemd.so.0.20.0:     file format elf64-x86-64
+build2/libsystemd.so.0.20.0:     file format elf64-x86-64