Add missing includes (#131)

* Add missing includes

This is causing problems with recent Xcode version on macOS.

* Enable macOS in GitHub Actions
diff --git a/.github/workflows/presubmit.yml b/.github/workflows/presubmit.yml
index 72ffef5..d520ae9 100644
--- a/.github/workflows/presubmit.yml
+++ b/.github/workflows/presubmit.yml
@@ -7,7 +7,7 @@
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04] # TODO: add macos-11.0
+        os: [ubuntu-20.04, macos-11.0]
 
     steps:
       - uses: actions/checkout@v2
diff --git a/loader/linux/icd_linux_envvars.c b/loader/linux/icd_linux_envvars.c
index 4af5a08..bfac87c 100644
--- a/loader/linux/icd_linux_envvars.c
+++ b/loader/linux/icd_linux_envvars.c
@@ -24,6 +24,7 @@
 #include "icd_cmake_config.h"
 
 #include <stdlib.h>
+#include <unistd.h>
 
 char *khrIcd_getenv(const char *name) {
     // No allocation of memory necessary for Linux.
diff --git a/test/loader_test/icd_test_match.c b/test/loader_test/icd_test_match.c
index a8330b4..c9207a3 100644
--- a/test/loader_test/icd_test_match.c
+++ b/test/loader_test/icd_test_match.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <stdlib.h>
 #include <string.h>
 #ifndef __APPLE__
 #include <malloc.h>