seccomp: Don't include libseccomp from QEMU header

The only prototype doesn't need anything from the lib header, and not
including it here allows files that include this header, for example
vl.c, to compile without the libseccomp cflags.

The breakage is since c3883e1f for environments where `pkg-config
--cflags libseccomp" is non-empty.
Reported-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Acked-by: NEduardo Otubo <otubo@redhat.com>
Message-id: 20170920083647.14599-1-famz@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

include/sysemu/seccomp.h

@@ -21,7 +21,5 @@
 #define QEMU_SECCOMP_SET_SPAWN       (1 << 3)
 #define QEMU_SECCOMP_SET_RESOURCECTL (1 << 4)
 
-#include <seccomp.h>
-
 int seccomp_start(uint32_t seccomp_opts);
 #endif