Do minor cleanups (#180)

Due to current shecc's implementation, modern editors with common lsp
such as VSCode with C/C++ plugin is unable to properly analyze from
main.c, even though the code is correct. Thus we adds some system header
inclusion in certain files to resolve and suppress error message. Notice
that due to current frontend's limitation, including defs.h in C source
files other than main.c would result weird error, thus this is not
introduced in this commit.

In addition, it's annoying that VSCode always generate .vscode folder
with some plugin-specific setting config files. Therefore, we also
ignore it to prevent accidentally adding the config file to stage.

.gitignore

@@ -8,3 +8,6 @@ a.out
 config
 src/codegen.c
 .session.mk
+
+# vscode C/C++ plugin generated files
+.vscode

src/globals.c

@@ -5,6 +5,9 @@
  * file "LICENSE" for information on usage and redistribution of this file.
  */
 
+#include <stdbool.h>
+#include <stdlib.h>
+
 /* Global objects */
 
 block_list_t BLOCKS;

src/lexer.c

@@ -5,6 +5,8 @@
  * file "LICENSE" for information on usage and redistribution of this file.
  */
 
+#include <stdbool.h>
+
 /* lexer tokens */
 typedef enum {
     T_start, /* FIXME: it was intended to start the state machine. */

src/parser.c

@@ -5,6 +5,9 @@
  * file "LICENSE" for information on usage and redistribution of this file.
  */
 
+#include <stdbool.h>
+#include <stdlib.h>
+
 /* C language syntactic analyzer */
 int global_var_idx = 0;
 int global_label_idx = 0;