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/libc/include/dlfcn.h b/libc/include/dlfcn.h
index a53f664..9aa4a1f 100644
--- a/libc/include/dlfcn.h
+++ b/libc/include/dlfcn.h
@@ -51,7 +51,7 @@
 
 void* dlopen(const char* filename, int flag);
 int dlclose(void* _Nonnull handle);
-const char* dlerror(void);
+char* dlerror(void);
 void* dlsym(void* handle, const char* _Nonnull symbol);
 void* dlvsym(void* handle, const char* _Nonnull symbol, const char* _Nonnull version) __INTRODUCED_IN(24);
 int dladdr(const void* addr, Dl_info* _Nonnull info);