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

Reviewed by:	jhb, markj, imp
Sponsored by:	vStack
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D35719

lib/libvmmapi/vmmapi.h

@@ -31,6 +31,7 @@
 #ifndef _VMMAPI_H_
 #define	_VMMAPI_H_
 
+#include <sys/cdefs.h>
 #include <sys/param.h>
 #include <sys/cpuset.h>
 #include <machine/vmm.h>
@@ -77,6 +78,7 @@ enum {
 	VM_PCIROM,
 };
 
+__BEGIN_DECLS
 /*
  * Get the length and name of the memory segment identified by 'segid'.
  * Note that system memory segments are identified with a nul name.
@@ -264,5 +266,6 @@ void	vm_setup_freebsd_gdt(uint64_t *gdtr);
  */
 int	vm_snapshot_req(struct vm_snapshot_meta *meta);
 int	vm_restore_time(struct vmctx *ctx);
+__END_DECLS
 
 #endif	/* _VMMAPI_H_ */