pkg/system: Set appropriate CFLAGS on Solaris

The cgo in `meminfo_solaris.go` initializes variables in for loops, and
therefore requires a `-std=c99` `CFLAG` in order to compile (at least on
modern SmartOS).

Signed-off-by: James Nugent <james@jen20.com>

pkg/system/meminfo_solaris.go

@@ -7,6 +7,7 @@ import (
 	"unsafe"
 )
 
+// #cgo CFLAGS: -std=c99
 // #cgo LDFLAGS: -lkstat
 // #include <unistd.h>
 // #include <stdlib.h>