bmake: fix -fno-common build

debug was declared extern, but debug_file was not; correct this and define
debug_file in main.c (as debug is) to fix the -fno-common build.

-fno-common will become the default with GCC10/LLVM11.

MFC after:	3 days

contrib/bmake/main.c

@@ -199,6 +199,8 @@ char *makeDependfile;
 pid_t myPid;
 int makelevel;
 
+FILE	*debug_file;
+
 Boolean forceJobs = FALSE;
 
 /*

contrib/bmake/make.h

@@ -464,7 +464,7 @@ extern pid_t	myPid;
  *	There is one bit per module.  It is up to the module what debug
  *	information to print.
  */
-FILE *debug_file;		/* Output written here - default stdout */
+extern FILE *debug_file;	/* Output written here - default stdout */
 extern int debug;
 #define	DEBUG_ARCH	0x00001
 #define	DEBUG_COND	0x00002