gh-39111: build/pkgs/planarity/spkg-configure.m4: update header check
    
In the `./configure` test for libplanarity, we should be testing for
`planarity/graph.h` instead of `planarity/planarity.h`:

  1. `graph.h` is what we actually use in
`src/sage/graphs/planarity.pyx`,
  2. `planarity.h` is going away in a future version.

cf. https://github.com/graph-algorithms/edge-addition-planarity-
suite/pull/125
    
URL: #39111
Reported by: Michael Orlitzky
Reviewer(s): Frédéric Chapoton

Author: Release Manager

build/pkgs/planarity/spkg-configure.m4

@@ -1,6 +1,6 @@
 SAGE_SPKG_CONFIGURE([planarity], [
      AC_LANG_PUSH([C])
-     AC_CHECK_HEADER([planarity/planarity.h], [
+     AC_CHECK_HEADER([planarity/graph.h], [
         AC_CHECK_LIB([planarity], [gp_InitGraph], [
          AC_MSG_CHECKING([for planarity version 3.0 or later])
          AC_COMPILE_IFELSE(