Makefile: Disable Windows targets

Windows builds currently fail with

    internal/daemon/instance/logstash/instance.go:94:45: unknown field
    'Setpgid' in struct literal of type syscall.SysProcAttr

and to be able to run "make release-tarballs" we disable the Windows
targets in the makefile. It remains to be decided whether we should fix
this error (probably by disabling daemon mode) or drop Windows support.

Makefile

@@ -31,7 +31,7 @@ PROGRAM := logstash-filter-verifier
 # List of all GOOS_GOARCH combinations that we should build release
 # binaries for. See https://golang.org/doc/install/source#environment
 # for all available combinations.
-TARGETS := darwin_amd64 linux_386 linux_amd64 windows_386 windows_amd64
+TARGETS := darwin_amd64 linux_386 linux_amd64
 
 VERSION := $(shell git describe --tags --always)