Fix warnings on VS2015 x86_64 build

gm/lightingshader.cpp

Index: gm/lightingshader.cpp
diff --git a/gm/lightingshader.cpp b/gm/lightingshader.cpp
index 687968abb68663d7a95085bc9b79473db4d0ddcf..771c0bc2e3b333d291c89e068187b61e8df053ba 100644
--- a/gm/lightingshader.cpp
+++ b/gm/lightingshader.cpp
@@ -38,7 +38,7 @@ static SkBitmap make_hemi_normalmap(int texSize) {
                 locY = 0.0f;
                 locZ = 0.0f;
             }
-            locZ = sqrt(1.0f - locZ);
+            locZ = sqrtf(1.0f - locZ);
             unsigned char r = static_cast<unsigned char>((0.5f * locX + 0.5f) * 255);
             unsigned char g = static_cast<unsigned char>((-0.5f * locY + 0.5f) * 255);
             unsigned char b = static_cast<unsigned char>((0.5f * locZ + 0.5f) * 255);