Fix with poppler-26.06.0.

From https://gitlab.com/inkscape/inkscape/-/work_items/6210,
1.4.x-Poppler-26.06.0_rev0.patch

Index: src/extension/internal/pdfinput/svg-builder.h
--- src/extension/internal/pdfinput/svg-builder.h.orig
+++ src/extension/internal/pdfinput/svg-builder.h
@@ -186,7 +186,7 @@ class SvgBuilder { (private)
     // Pattern creation
     gchar *_createPattern(GfxPattern *pattern, GfxState *state, bool is_stroke=false);
     gchar *_createGradient(GfxShading *shading, const Geom::Affine pat_matrix, bool for_shading = false);
-    void _addStopToGradient(Inkscape::XML::Node *gradient, double offset, GfxColor *color, GfxColorSpace *space,
+    void _addStopToGradient(Inkscape::XML::Node *gradient, double offset, GfxColor &color, GfxColorSpace *space,
                             double opacity);
     bool _addGradientStops(Inkscape::XML::Node *gradient, GfxShading *shading,
                            _POPPLER_CONST Function *func);
@@ -239,6 +239,7 @@ class SvgBuilder { (private)
     static bool _attrEqual(Inkscape::XML::Node *a, Inkscape::XML::Node *b, char const *attr);
 
     // Colors
+    std::string convertGfxColor(const GfxColor &color, GfxColorSpace *space);
     std::string convertGfxColor(const GfxColor *color, GfxColorSpace *space);
     std::string _getColorProfile(cmsHPROFILE hp);
 
