Fix library link in doxygen docs for image handlers

The command used for linking to the core library was "@library{core}"
instead of correct "@library{wxcore}". This resulted into plain text
"page_libs_core" being displayed instead of expected "wxCore" hyperlink.

Closes https://github.com/wxWidgets/wxWidgets/pull/1259

interface/wx/imaggif.h

@@ -12,7 +12,7 @@
 
     This is the image handler for the GIF format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagiff.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the IFF format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagjpeg.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the JPEG format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagpcx.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the PCX format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagpng.h

@@ -30,7 +30,7 @@ enum
 
     This is the image handler for the PNG format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagpnm.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the PNM format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagtga.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the TGA format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()

interface/wx/imagxpm.h

@@ -10,7 +10,7 @@
 
     This is the image handler for the XPM format.
 
-    @library{core}
+    @library{wxcore}
     @category{gdi}
 
     @see wxImage, wxImageHandler, wxInitAllImageHandlers()