Move rs.h functions over to extern "C".

Change-Id: I0503a64d93dda558455dfebca90a0c820c80d7b1
diff --git a/rsDevice.cpp b/rsDevice.cpp
index d7d03f6..2688890 100644
--- a/rsDevice.cpp
+++ b/rsDevice.cpp
@@ -40,17 +40,17 @@
     }
 }
 
-RsDevice rsDeviceCreate() {
+extern "C" RsDevice rsDeviceCreate() {
     Device * d = new Device();
     return d;
 }
 
-void rsDeviceDestroy(RsDevice dev) {
+extern "C" void rsDeviceDestroy(RsDevice dev) {
     Device * d = static_cast<Device *>(dev);
     delete d;
 }
 
-void rsDeviceSetConfig(RsDevice dev, RsDeviceParam p, int32_t value) {
+extern "C" void rsDeviceSetConfig(RsDevice dev, RsDeviceParam p, int32_t value) {
     Device * d = static_cast<Device *>(dev);
     if (p == RS_DEVICE_PARAM_FORCE_SOFTWARE_GL) {
         d->mForceSW = value != 0;