Fix msvc build

Apparantly, msvc can't handle defaulted constructors where the class
contains a incomplete type in a unique_ptr.

Move the defaulted constructor implementation to cpp file to ensure it
is no longer a incomplete type
diff --git a/poppler/SignatureInfo.cc b/poppler/SignatureInfo.cc
index 5eb1443..14e9c88 100644
--- a/poppler/SignatureInfo.cc
+++ b/poppler/SignatureInfo.cc
@@ -28,6 +28,7 @@
 /* Constructor & Destructor */
 
 SignatureInfo::~SignatureInfo() = default;
+SignatureInfo::SignatureInfo() = default;
 
 /* GETTERS */
 
diff --git a/poppler/SignatureInfo.h b/poppler/SignatureInfo.h
index 62374ba..8e5ad37 100644
--- a/poppler/SignatureInfo.h
+++ b/poppler/SignatureInfo.h
@@ -54,7 +54,7 @@
 class POPPLER_PRIVATE_EXPORT SignatureInfo
 {
 public:
-    SignatureInfo() = default;
+    SignatureInfo();
     ~SignatureInfo();
 
     SignatureInfo(const SignatureInfo &) = delete;