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. graph-algorithms/edge-addition-planarity-suite#125

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(