build: Use install_man

There is a function called `install_man`[0] that installes man files
in their proper directory, so it has been used to install the man
file.

[0] https://mesonbuild.com/Reference-manual.html#install_man

Author: inigomartinez

data/meson.build

@@ -1,8 +1,5 @@
 # Install man page
-install_data(
-  'meld.1',
-  install_dir: mandir / 'man1',
-)
+install_man('meld.1')
 
 # Install schema file
 install_data(