makefile: fix qemu-ga make install for --disable-tools

ab59e3e introduced a fix for `make install` on w32 that involved
filtering out qemu-ga from $TOOLS install recipe so that we could
append $(EXESUF) to it before attempting to install the binary
via install-prog function.

install-prog takes a list of binaries to install to a particular
directory. If the list is empty it breaks. We guard against this
by ensuring $TOOLS is not empty prior to calling.

However, ab59e3e introduces extra filtering after this check which
can still result on us attempting to call install-prog with an
empty list of binaries. In particular, this occurs if we
build with the --disable-tools configure option, which results
in qemu-ga being the only member of $TOOLS.

Fix this by doing a simple s/qemu-ga/qemu-ga$(EXESUF)/ pass through
$TOOLS instead of filtering out qemu-ga to handle it seperately.

Reported-by: Steve Ellcey <sellcey@imgtec.com>
Cc: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
