core plugin: suppress 'penwidth' double comparison warning

core plugin: suppress 'penwidth' double comparison warning

`gvprintdouble` prints to two decimal places. So we can take this into account
when deciding whether we need to print a non-default pen width, squashing:

  gvrender_core_svg.c: In function ‘svg_grstyle’:
  gvrender_core_svg.c:191:23: warning: comparing floating-point with ‘==’ or
    ‘!=’ is unsafe [-Wfloat-equal]
    191 |     if (obj->penwidth != PENWIDTH_NORMAL) {
        |                       ^~