target: don't export local symbols

target: don't export local symbols

Symbols that are not used outside the file should not be exported
and should be declared as static.

Change-Id: Icbe7f7bce287b903edec9dc9db3370722c51fbd5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7173
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>