mysql-python: add dependency on mysql

* fails without mysql_config:
  ERROR: Function failed: do_compile (log file is located at mysql-python/1.2.4-r0/temp/log.do_compile.29833)
  ERROR: Logfile of failure stored in: mysql-python/1.2.4-r0/temp/log.do_compile.29833
  Log data follows:
  | DEBUG: Executing shell function do_compile
  | sh: 1: mysql_config: not found
  | Traceback (most recent call last):
  |   File "setup.py", line 18, in <module>
  |     metadata, options = get_config()
  |   File "mysql-python/1.2.4-r0/MySQL-python-1.2.4/setup_posix.py", line 43, in get_config
  |     libs = mysql_config("libs_r")
  |   File "mysql-python/1.2.4-r0/MySQL-python-1.2.4/setup_posix.py", line 25, in mysql_config
  |     raise EnvironmentError("%s not found" % (mysql_config.path,))
  | EnvironmentError: mysql_config not found
  | ERROR: python setup.py build_ext execution failed.
  | ERROR: Function failed: do_compile (log file is located at mysql-python/1.2.4-r0/temp/log.do_compile.29833)
  NOTE: recipe mysql-python-1.2.4-r0: task do_compile: Failed
  ERROR: Task 6 (meta-oe/recipes-support/mysql/mysql-python_1.2.4.bb, do_compile) failed with exit code '1'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>