Fix build with ToT libc++

packages/modules/adb/sysdeps/env.cpp:54:23: error: use of undeclared
identifier 'getenv'
   54 |     const char* val = getenv(var.data());
      |

Test: Build with ToT libc++
Change-Id: Ic23a64add8a5f73850ddd8663dd91be7001abb92
diff --git a/sysdeps/env.cpp b/sysdeps/env.cpp
index 4058728..82d553a 100644
--- a/sysdeps/env.cpp
+++ b/sysdeps/env.cpp
@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+#include <stdlib.h>
+
 #include "sysdeps/env.h"
 
 #ifdef _WIN32