sr/srd: SR_API/SRD_API/etc. is a no-op on MinGW.

This avoids warnings on a MinGW build. The feature depends on ELF files,
but MinGW/Windows uses PE files.

sigrok.h.in

@@ -115,14 +115,25 @@ extern "C" {
  *
  * This uses the 'visibility' feature of gcc (requires gcc >= 4.0).
  *
+ * This feature is not available on MinGW/Windows, as it is a feature of
+ * ELF files and MinGW/Windows uses PE files.
+ *
  * Details: http://gcc.gnu.org/wiki/Visibility
  */
 
 /* Marks public libsigrok API symbols. */
+#ifndef _WIN32
 #define SR_API __attribute__((visibility("default")))
+#else
+#define SR_API
+#endif
 
 /* Marks private, non-public libsigrok symbols (not part of the API). */
+#ifndef _WIN32
 #define SR_PRIV __attribute__((visibility("hidden")))
+#else
+#define SR_PRIV
+#endif
 
 typedef int (*sr_receive_data_callback_t)(int fd, int revents, void *cb_data);