Fixed build when PATH contains spaces (#140)

Fixes issue introduced in #63 - if PATH contains spaces (or special characters) when this line of the Makefile runs, it will corrupt the PATH variable and the build will fail.
Putting quotes around the result of the string substitution fixes this.

Author: RussellJoyce

libmicroros.mk

@@ -85,7 +85,7 @@ $(EXTENSIONS_DIR)/micro_ros_src/src:
 $(EXTENSIONS_DIR)/micro_ros_src/install: $(EXTENSIONS_DIR)/esp32_toolchain.cmake $(EXTENSIONS_DIR)/micro_ros_dev/install $(EXTENSIONS_DIR)/micro_ros_src/src
 	cd $(UROS_DIR); \
 	unset AMENT_PREFIX_PATH; \
-	PATH=$(subst /opt/ros/$(ROS_DISTRO)/bin,,$(PATH)); \
+	PATH="$(subst /opt/ros/$(ROS_DISTRO)/bin,,$(PATH))"; \
 	. ../micro_ros_dev/install/local_setup.sh; \
 	colcon build \
 		--merge-install \