Add grpc_posix.h header for grpc_use_signal declaration
diff --git a/src/core/lib/iomgr/ev_epollsig_linux.cc b/src/core/lib/iomgr/ev_epollsig_linux.cc
index 370ea1d..035bdc4 100644
--- a/src/core/lib/iomgr/ev_epollsig_linux.cc
+++ b/src/core/lib/iomgr/ev_epollsig_linux.cc
@@ -18,6 +18,8 @@
 
 #include "src/core/lib/iomgr/port.h"
 
+#include <grpc/grpc_posix.h>
+
 /* This polling engine is only relevant on linux kernels supporting epoll() */
 #ifdef GRPC_LINUX_EPOLL