Fix memory leak in buildExtensionString

Fixes: 263408475
Test: `atest hwui_unit_tests` doesn't report a bunch of leaks
Change-Id: I5ad6ac50ff08cd1a3b3e6bf96465f7d5db3eedcd
diff --git a/system/egl/eglDisplay.cpp b/system/egl/eglDisplay.cpp
index 3950ffd..79019c8 100644
--- a/system/egl/eglDisplay.cpp
+++ b/system/egl/eglDisplay.cpp
@@ -367,6 +367,7 @@
 
         asprintf(&finalEGLExts, "%s%s", initialEGLExts, dynamicEGLExtensions.c_str());
 
+        free(initialEGLExts);
         free((char*)hostExt);
         return finalEGLExts;
     }