bitmap-str: Get rid of 'extern' for function prototypes

The bitmap-str.h uses mixed style for function prototypes. Drop
the 'extern' as it easier to read and makes style aligned with
a new code in the kernel.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>