libgap_set_error() not in libgap.h but called by libgap_enter()

[Jeroen Demeyer]

libgap.h has:

#define libgap_enter()                       \
  if (libgap_in_enter_exit_block) {              \
    libgap_set_error("Entered a critical block twice");      \
    libgap_call_error_handler();                 \
  }                              \
  libgap_in_enter_exit_block = 1;                \
  libgap_mark_stack_bottom();

but libgap_set_error() is not actually declared since the prototype is in libgap_internal.h. It is still valid C89, but not valid C99 or C++. GCC give a warning

build/cythonized/sage/libs/gap/util.c: In function ‘__pyx_f_4sage_4libs_3gap_4util_initialize’:
build/cythonized/sage/libs/gap/util.c:4578:3: warning: implicit declaration of function ‘libgap_set_error’ [-Wimplicit-function-declaration]
   libgap_enter();
   ^