Add submodule requirement to makefile

Makefile

@@ -3,6 +3,7 @@ BUILD_DIR ?= build
 SRC_MAIN  ?= $(SRC_DIR)/main.c
 EXE_NAME  ?= fbtest
 BIN       := $(BUILD_DIR)/$(EXE_NAME)
+BASE_UXN  := src/uxn/src/uxn.h
 
 CC        ?= cc
 CFLAGS    := -Wall -Wextra -pedantic
@@ -23,12 +24,16 @@ endif
 
 main: $(BIN)
 
-$(BIN): $(SRC_MAIN) $(BUILD_DIR)
+$(BIN): $(SRC_MAIN) $(BUILD_DIR) $(BASE_UXN)
 	$(CC) $(CFLAGS) -o $(BIN) $(SRC_MAIN)
 
 $(BUILD_DIR):
 	mkdir -p $(BUILD_DIR)
 
+$(BASE_UXN):
+	git submodule init
+	git submodule update
+
 run: $(BIN)
 	# NOTE: This should probably be done on the C code.
 	echo 0 > /sys/class/graphics/fbcon/cursor_blink