pkgconfig: avoid double slash with split-usr configuration

pkgconfig: avoid double slash with split-usr configuration

By defining rootprefix= we avoid a double slash in $systemdsystemunitdir and
other variables. This fixes a regression introduced in
1c2c7c6cb3d92315624f3711114e86b0acfbce63 where the variables using rootprefix=/
would start with a double slash. This should be interpreted the same, but is
certainly ugly.

The rootprefix variable was added to systemd.pc in
1c2c7c6cb3d92315624f3711114e86b0acfbce63, so there is no question of backwards
compatiblity. If people try to "override" the prefix and specify
--define-variable=rootprefix=/, they will get a double slash, which should be
OK, and is the same as --define-variable=rootprefix=/something/, which also
results in a double slash somewhere in the strings.