helper: Remove src/helper from include dirs

The header files under src/helper/ can currently be included with
either

    #include <bits.h>
    or
    #include <helper/bits.h>

This is because we specify both "src/" and "src/helper/" directories
as include directories.  Some files name under "src/helper/", such as
types.h, log.h, and util.h are too generic and could be ambiguous
depending on the search path.

This commit remove "src/helper/" from our include dir and make C files
include explicitly.

Change-Id: I38fc9b96ba01a513d4a72757d40007e21b502f25
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6507
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/target/riscv/riscv_semihosting.c

@@ -41,7 +41,7 @@
 #include "config.h"
 #endif
 
-#include "log.h"
+#include <helper/log.h>
 
 #include "target/target.h"
 #include "target/semihosting_common.h"