[wpimath] Explicitly export wpimath symbols

Co-authored-by: Tyler Veness <calcmogul@gmail.com>

wpiutil/src/main/native/include/wpi/SymbolExports.h

@@ -0,0 +1,23 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#ifdef _WIN32
+#pragma warning(disable : 4251)
+
+#ifdef WPILIB_EXPORTS
+#ifdef __GNUC__
+#define WPILIB_DLLEXPORT __attribute__((dllexport))
+#else
+#define WPILIB_DLLEXPORT __declspec(dllexport)
+#endif
+
+#else
+#define WPILIB_DLLEXPORT
+#endif
+
+#else
+#define WPILIB_DLLEXPORT __attribute__((visibility("default")))
+#endif

wpiutil/src/main/native/include/wpi/sendable/Sendable.h

@@ -4,6 +4,8 @@
 
 #pragma once
 
+#include "wpi/SymbolExports.h"
+
 namespace wpi {
 
 class SendableBuilder;
@@ -11,7 +13,7 @@ class SendableBuilder;
 /**
  * Interface for Sendable objects.
  */
-class Sendable {
+class WPILIB_DLLEXPORT Sendable {
  public:
   virtual ~Sendable() = default;