Added BEGIN_DECLS and END_DECLS to stm32/common/gpio_common_all.h and stm32/l1/rcc.h

Fixed:
stm32/common/gpio_common_all.h
stm32/l1/rcc.h

Added BEGIN_DECLS and END_DECLS to function prototypes list to avoid the following errors when use funcs from c++:

undefined reference to `gpio_set(unsigned long, unsigned short)'
collect2: error: ld returned 1 exit status
make: *** [stm32lib_cpp_dev_f2x.elf] Error 1