dlerror returns char*, not const char*.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/dlerror.html:

    char *dlerror(void);
    ...
    The application shall not modify the string returned.

Change-Id: I5e684bfd3930c39a2a30ea6fd005a5d5d3e5b181
diff --git a/libdl/libdl.c b/libdl/libdl.c
index b62ee5c..4cc4dea 100644
--- a/libdl/libdl.c
+++ b/libdl/libdl.c
@@ -25,7 +25,7 @@
 
 void* dlopen(const char* filename __unused, int flag __unused) { return 0; }
 
-const char* dlerror(void) { return 0; }
+char* dlerror(void) { return 0; }
 
 void* dlsym(void* handle __unused, const char* symbol __unused) { return 0; }