libvmm: add __BEGIN_DECLS/__END_DECLS for linking with c++ binaries

[gusev.vitaliy_gmail.com]

Summary

Sponsored by vStack.

Test Plan

$ cat vmopen.cpp

#include <dlfcn.h>
#include <err.h>
#include <vmmapi.h>

typedef void (*vm_close_fn)(struct vmctx *ctx);
int main(int argc, char *argv[])
{
	const char *vmname = argc > 1 ? argv[1] : "-";
	struct vmctx *ctx;
	vm_close_fn __vm_close = (vm_close_fn)dlsym(RTLD_NEXT, "vm_close");

	ctx = vm_open(vmname);

	if (ctx == NULL)
		err(1, "cannot open vm device: %s", vmname);

	if (__vm_close)
		__vm_close(ctx);
}

Compilation and linking

$ c++ vmopen.cpp -I /data/freebsd/HEAD/lib/libvmmapi/ -L /data/freebsd/HEAD/lib/libvmmapi/ -lvmmapi

Without patch command above produces:

ld: error: undefined symbol: vm_open(char const*)

With patch - success.

[markj]

lib/libvmmapi/vmmapi.h

36

Conventionally, cdefs.h is included first (see style(9)). So:

#include <sys/cdefs.h>
#include <sys/param.h>
#include ...

[gusev.vitaliy_gmail.com]

Corrected include order according to Mark's comment.

[gusev.vitaliy_gmail.com]

@markj Could you approve again and commit?