Fix broken fuchsia build.

Caused by:
https://chromium-review.googlesource.com/1022186

Bug: internal b/79235281
Test: local build
Change-Id: I681c8aa74aa8d374fadf31f136d7c2367a978d3c
Reviewed-on: https://chromium-review.googlesource.com/1044486Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Reviewed-by: Stephen Lanham <slan@chromium.org>
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Commit-Queue: Gaofeng Huang <gfhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556475}

components/os_crypt/BUILD.gn

@@ -51,7 +51,8 @@ static_library("os_crypt") {
     "//crypto:platform",
   ]
 
-  if (is_posix && !is_mac && !is_ios && (!is_desktop_linux || is_chromecast)) {
+  if ((is_posix || is_fuchsia) && !is_mac && !is_ios &&
+      (!is_desktop_linux || is_chromecast)) {
     sources += [ "os_crypt_posix.cc" ]
   }