Remove alias syntax to support macOS build

[tidefield]

When building for macOS as part of envoyproxy/envoy#40967, the compiler yields an error

external/com_github_openhistogram_libcircllhist/src/circllhist.c:685:25: error: aliases are not supported on darwin
685 | uint64_t attribute((alias("hist_approx_count_below_inclusive"))) hist_approx_count_below(const histogram_t *hist, double threshold);
| ^
external/com_github_openhistogram_libcircllhist/src/circllhist.c:686:25: error: aliases are not supported on darwin
686 | uint64_t attribute((alias("hist_approx_count_above_inclusive"))) hist_approx_count_above(const histogram_t *hist, double threshold);

This PR simply replaces the alias syntax with the same implementation.

[esproul]

LGTM, thanks for your contribution!