[PATCH] quiet conversion warning in DatumGetFloat4

It seems that 14cca1b (use static inline functions for float <-> Datum
conversions) has an implicit narrowing conversion in one of those
functions.

If building an extension with gcc's -Wconversion warning enabled
(*cough* pljava *cough* ... the Maven plugin that runs the compiler
enables the warning by default), this makes for a noisy build.
The warning is harmless, but repeated everywhere postgres.h is
included. An explicit cast is enough to suppress it.

-Chap