cmake add a few include guards

- this is a minor cmake configure time optimization

cmake/px4_find_python_module.cmake

@@ -31,6 +31,12 @@
 #
 ############################################################################
 
+# cmake include guard
+if(px4_find_python_module_included)
+	return()
+endif(px4_find_python_module_included)
+set(px4_find_python_module_included true)
+
 #
 #	px4_find_python_module
 #

cmake/px4_git.cmake

@@ -31,6 +31,12 @@
 #
 ############################################################################
 
+# cmake include guard
+if(px4_git_included)
+	return()
+endif(px4_git_included)
+set(px4_git_included true)
+
 #=============================================================================
 #
 #	px4_add_git_submodule

cmake/px4_parse_function_args.cmake

@@ -31,6 +31,12 @@
 #
 ############################################################################
 
+# cmake include guard
+if(px4_parse_function_args_included)
+	return()
+endif(px4_parse_function_args_included)
+set(px4_parse_function_args_included true)
+
 #=============================================================================
 #
 #	px4_parse_function_args