Use __builtin_tgamma instead of tgamma.

In C++, tgamma is an overloaded macro referring to tgammaf.

This wasn't surfaced until now because the old NDK libc++ didn't have
its own math.h.

Test: mma
Bug: None
Change-Id: I32f75036bb1118f71ec7e0fe17220523b7eacb8f
diff --git a/cpu_ref/rsCpuRuntimeMath.cpp b/cpu_ref/rsCpuRuntimeMath.cpp
index 24539d3..b416462 100644
--- a/cpu_ref/rsCpuRuntimeMath.cpp
+++ b/cpu_ref/rsCpuRuntimeMath.cpp
@@ -47,7 +47,7 @@
 // Handle missing Gingerbread functions like tgammaf.
 float SC_tgammaf(float x) {
 #ifdef RS_COMPATIBILITY_LIB
-    return tgamma(x);
+    return __builtin_tgamma(x);
 #else
     return tgammaf(x);
 #endif