build: ignore cygwin toolchain droppings

build: ignore cygwin toolchain droppings

The cygwin compiler automatically creates a '*.exe.manifest'
companion file for any .exe file that contains a substring
that would otherwise cause newer Windows to pester users about
needing admin rights (such as "update", "instal", "setup"...).
This means that compilation on cygwin left behind
tests/networkxml2xmlupdatetest.exe.manifest.

* .gitignore: Ignore manifest files.

Signed-off-by: Eric Blake <eblake@redhat.com>