Help: Guard calls to pkg_check_modules in cmake-developer(7) example

This was missed in commit 25b94758 (Modules: Guard calls to
pkg_check_modules, 2024-08-07, v3.31.0-rc1~236^2).

Help/manual/cmake-developer.7.rst

@@ -408,7 +408,9 @@ starting point.
 .. code-block:: cmake
 
   find_package(PkgConfig)
-  pkg_check_modules(PC_Foo QUIET Foo)
+  if(PKG_CONFIG_FOUND)
+    pkg_check_modules(PC_Foo QUIET Foo)
+  endif()
 
 This should define some variables starting ``PC_Foo_`` that contain the
 information from the ``Foo.pc`` file.