plugin: cast dlerror return value for android

Until a few weeks ago, bionic, the Andoid libc, incorrectly
returned const char* (instead of char*) from dlerror(3).

https://android.googlesource.com/platform/bionic/+/5e071a18ce88d93fcffaebb9e0f62524ae504908

Change-Id: I30d33240c63a9f35b6c20ca7e3928ad33bc5e33f
Reviewed-on: https://go-review.googlesource.com/29352
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/plugin/plugin_dlopen.go

@@ -16,7 +16,7 @@ package plugin
 static uintptr_t pluginOpen(const char* path, char** err) {
 	void* h = dlopen(path, RTLD_NOW|RTLD_GLOBAL);
 	if (h == NULL) {
-		*err = dlerror();
+		*err = (char*)dlerror();
 	}
 	return (uintptr_t)h;
 }
@@ -24,7 +24,7 @@ static uintptr_t pluginOpen(const char* path, char** err) {
 static void* pluginLookup(uintptr_t h, const char* name, char** err) {
 	void* r = dlsym((void*)h, name);
 	if (r == NULL) {
-		*err = dlerror();
+		*err = (char*)dlerror();
 	}
 	return r;
 }