meson: Fix pc file generation

The previous commit adding meson support forgot to add the gpioplus
library to the pkgconfig file, so anything depending on gpioplus
wouldn't get the linker flag that is required to link against it.

Change-Id: Id0ed6dabede79bd137ae5f56806e7202f8a238c9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/meson.build b/src/meson.build
index 056a1cd..9234276 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -16,7 +16,7 @@
   name: 'gpioplus',
   description: 'C++ systemd event wrapper',
   version: meson.project_version(),
-)
+  libraries: gpioplus)
 
 install_headers(
   'gpioplus/chip.hpp',