Fix libinput warnings

The warnings were being hidden by the use of -isystem to include
frameworks/native/include.

Bug: 31752268
Test: m -j
Change-Id: I13f03892c75ea3c3139ca676666b0d403871d241
diff --git a/include/input/Input.h b/include/input/Input.h
index 55787e7..cfcafab 100644
--- a/include/input/Input.h
+++ b/include/input/Input.h
@@ -398,7 +398,7 @@
 
     inline int32_t getButtonState() const { return mButtonState; }
 
-    inline int32_t setButtonState(int32_t buttonState) { mButtonState = buttonState; }
+    inline void setButtonState(int32_t buttonState) { mButtonState = buttonState; }
 
     inline int32_t getActionButton() const { return mActionButton; }