makefile: fix w32 install target for qemu-ga

fafcaf1d added a 'qemu-ga' install target on w32, which can be used
in place of the existing qemu-ga.exe target to also handle dealing
with other components such as DLLs for VSS/fsfreeze and generating
an MSI package if appropriate configure options are present.

As part of that, qemu-ga$(EXESUF) was removed from $TOOLS in favor
of this new qemu-ga target.

The install rule however relies on a direct mapping of the $TOOLS
entry to the actual resulting binary. In the case of w32, qemu-ga
is not identical to qemu-ga$(EXESUF), and the install recipe fails
to find the 'qemu-ga' binary.

Fix this by essentially remapping 'qemu-ga' back to 'qemu-ga.exe'
in the install recipe.

This raises the question of whether or not qemu-ga should continue
to live in TOOLS as opposed to its own special target, but as a
late fix for a regression in 2.5 this commit should be safer, since
we rely on qemu-ga's presence in $TOOLS in several places throughout
Makefile.

Reported-by: Stefan Weil <sw@weilnetz.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>