mu: use g_strerror instead of strerror

Some #includes were missing for the latter (but only noticeable on some
systems - e.g., build breaks on Cygwin).

So let's replace with something that works equally everywhere.

Fixes: #2060

mu/mu-cmd-index.cc

@@ -59,7 +59,7 @@ install_sig_handler (void)
 	for (i = 0; i != G_N_ELEMENTS(sigs); ++i)
 		if (sigaction (sigs[i], &action, NULL) != 0)
 			g_critical ("set sigaction for %d failed: %s",
-				    sigs[i], strerror (errno));;
+				    sigs[i], g_strerror (errno));;
 }
 
 
@@ -101,7 +101,7 @@ Mu::mu_cmd_index (Mu::Store& store, const MuConfig *opts, GError **err)
         const auto mdir{store.metadata().root_maildir};
         if (G_UNLIKELY(access (mdir.c_str(), R_OK) != 0)) {
                 mu_util_g_set_error(err, MU_ERROR_FILE,
-                                   "'%s' is not readable: %s", mdir.c_str(), strerror (errno));
+                                   "'%s' is not readable: %s", mdir.c_str(), g_strerror (errno));
                 return MU_ERROR;
         }
 

mu/mu-cmd.cc

@@ -308,7 +308,7 @@ check_file_okay (const char *path, gboolean cmd_add)
 
 	if (cmd_add && access(path, R_OK) != 0) {
 		g_printerr ("path is not readable: %s: %s\n",
-                            path, strerror (errno));
+                            path, g_strerror (errno));
 		return FALSE;
 	}
 

mu/test-mu-cmd.cc

@@ -371,7 +371,7 @@ get_file_size (const char* path)
 
 	rv = stat (path, &statbuf);
 	if (rv != 0) {
-		/* g_warning ("error: %s", strerror (errno)); */
+		/* g_warning ("error: %s", g_strerror (errno)); */
 		return -1;
 	}