Overhaul scaninc to work recursively

This also fixes the bug where scaninc would ignore #include lines
in assembly files.

tools/scaninc/Makefile

@@ -2,9 +2,9 @@ CXX = g++
 
 CXXFLAGS = -Wall -Werror -std=c++11 -O2
 
-SRCS = scaninc.cpp c_file.cpp asm_file.cpp
+SRCS = scaninc.cpp c_file.cpp asm_file.cpp source_file.cpp
 
-HEADERS := scaninc.h asm_file.h c_file.h
+HEADERS := scaninc.h asm_file.h c_file.h source_file.h
 
 .PHONY: clean