Fix compilation on the macOS platform

This MR fixing babl build problems on Darwin. Logs for the ninja test are provided below:

bash-3.2$ ninja test
[1/2] Running all tests.
 1/23 babl_class_name                         OK       0.07 s
 2/23 cairo_cmyk_hack                         OK       0.11 s
 3/23 cairo-RGB24                             OK       0.38 s
 4/23 chromaticities                          OK       0.17 s
 5/23 extract                                 OK       0.09 s
 6/23 floatclamp                              OK       0.08 s
 7/23 float-to-8bit                           OK       0.72 s
 8/23 format_with_space                       OK       0.05 s
 9/23 grayscale_to_rgb                        OK       0.08 s
10/23 hsl                                     OK       0.04 s
11/23 hsva                                    OK       0.04 s
12/23 models                                  OK       0.07 s
13/23 n_components                            OK       0.39 s
14/23 n_components_cast                       OK       0.18 s
15/23 nop                                     OK       0.05 s
16/23 palette                                 OK       1.56 s
17/23 rgb_to_bgr                              OK       0.08 s
18/23 rgb_to_ycbcr                            OK       0.08 s
19/23 sanity                                  OK       0.04 s
20/23 srgb_to_lab_u8                          OK       0.08 s
21/23 transparent                             OK       2.91 s
22/23 alpha_symmetric_transform               OK       0.08 s
23/23 types                                   OK       0.04 s

Ok:                   23
Expected Fail:         0
Fail:                  0
Unexpected Pass:       0
Skipped:               0
Timeout:               0

Full log written to /Users/oleksii_samorukov/src/babl/_build/meson-logs/testlog.txt