Fix public headers for building in Visual Studio

Ensure that we also have import/export decorations for the prototypes, as
Visual Studio does not like such decorations on the function definitions
without the prototypes decorated as well, when the function prototypes
exist.

include/epoxy/gl.h

@@ -93,9 +93,9 @@ extern "C" {
 
 #include "epoxy/gl_generated.h"
 
-bool epoxy_has_gl_extension(const char *extension);
-bool epoxy_is_desktop_gl(void);
-int epoxy_gl_version(void);
+EPOXY_IMPORTEXPORT bool epoxy_has_gl_extension(const char *extension);
+EPOXY_IMPORTEXPORT bool epoxy_is_desktop_gl(void);
+EPOXY_IMPORTEXPORT int epoxy_gl_version(void);
 
 #ifdef __cplusplus
 } /* extern "C" */

include/epoxy/wgl.h

@@ -54,8 +54,8 @@ extern "C" {
 
 #include "epoxy/wgl_generated.h"
 
-bool epoxy_has_wgl_extension(HDC hdc, const char *extension);
-void epoxy_handle_external_wglMakeCurrent(void);
+EPOXY_IMPORTEXPORT bool epoxy_has_wgl_extension(HDC hdc, const char *extension);
+EPOXY_IMPORTEXPORT void epoxy_handle_external_wglMakeCurrent(void);
 
 #ifdef __cplusplus
 } /* extern "C" */