sddm: ignore config file modification time

Properly fixes #10211, wherein the config file was not being loaded
because it was considered too old. However, the modification time was
being compared against an uninitialized value; the semantics of the
uninitialized timestamp probably changed between Qt 5.4 and 5.5.

pkgs/applications/display-managers/sddm/default.nix

@@ -14,6 +14,8 @@ stdenv.mkDerivation rec {
     sha256 = "09amr61srvl52nvxlqqgs9fzn33pc2gjv5hc83gxx43x6q2j19gg";
   };
 
+  patches = [ ./sddm-ignore-config-mtime.patch ];
+
   nativeBuildInputs = [ cmake pkgconfig qttools ];
 
   buildInputs = [ libxcb libpthreadstubs libXdmcp libXau qtbase qtdeclarative pam systemd ];

pkgs/applications/display-managers/sddm/sddm-ignore-config-mtime.patch

@@ -0,0 +1,16 @@
+diff --git a/src/common/ConfigReader.cpp b/src/common/ConfigReader.cpp
+index 6618455..5356e76 100644
+--- a/src/common/ConfigReader.cpp
++++ b/src/common/ConfigReader.cpp
+@@ -136,11 +136,6 @@ namespace SDDM {
+         QString currentSection = QStringLiteral(IMPLICIT_SECTION);
+ 
+         QFile in(m_path);
+-        QDateTime modificationTime = QFileInfo(in).lastModified();
+-        if (modificationTime <= m_fileModificationTime) {
+-            return;
+-        }
+-        m_fileModificationTime = modificationTime;
+ 
+         in.open(QIODevice::ReadOnly);
+         while (!in.atEnd()) {