[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98597)

This is a part of #97655.
diff --git a/libc/src/math/fminf128.h b/libc/src/math/fminf128.h
index d2ed593..7bc393a 100644
--- a/libc/src/math/fminf128.h
+++ b/libc/src/math/fminf128.h
@@ -9,12 +9,13 @@
 #ifndef LLVM_LIBC_SRC_MATH_FMINF128_H
 #define LLVM_LIBC_SRC_MATH_FMINF128_H
 
+#include "src/__support/macros/config.h"
 #include "src/__support/macros/properties/types.h"
 
-namespace LIBC_NAMESPACE {
+namespace LIBC_NAMESPACE_DECL {
 
 float128 fminf128(float128 x, float128 y);
 
-} // namespace LIBC_NAMESPACE
+} // namespace LIBC_NAMESPACE_DECL
 
 #endif // LLVM_LIBC_SRC_MATH_FMINF128_H