objtool: Improve error message for bad file argument

If a nonexistent file is supplied to objtool, it complains with a
non-helpful error:

open: No such file or directory

Improve it to:

objtool: Can't open 'foo': No such file or directory

Reported-by: Markus <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

tools/objtool/elf.c

···
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include <errno.h>
 
 #include "elf.h"
 #include "warn.h"
···
 
 	elf->fd = open(name, flags);
 	if (elf->fd == -1) {
-		perror("open");
+		fprintf(stderr, "objtool: Can't open '%s': %s\n",
+			name, strerror(errno));
 		goto err;
 	}