pluma-metadata-manager: remove time.h

[rbuj]

Test:

1. disable gvfs metadata using --disable-gvfs-metadata

   ./autogen.sh --prefix=/usr --enable-debug --disable-gvfs-metadata && make && sudo make install

2. open a file in pluma, click anywhere in the opened file, and close pluma
3. check the value of atime in ~/.cache/pluma/pluma-metadata.xml

   <?xml version="1.0"?>
   <metadata>
   <document uri="file:///home/robert/backup.sh" atime="1637781907">
   <entry key="metadata::pluma-position" value="839"/>
   </document>
   </metadata>