Fix MSVC warning in compat_informix/rnull.pgc

Building the ecpg tests with MSVC, with warnings enabled, results in the
following warning:
src/interfaces/ecpg/test/compat_informix/rnull.pgc(19,1): warning C4305: 'initializing': truncation from 'double' to 'float'

The more obvious fix would be an 'f' suffix, but ecpg can't parse that.

Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/2180a97c-c026-1b6c-cec8-d6e499f97017@enterprisedb.com

src/interfaces/ecpg/test/compat_informix/rnull.pgc

@@ -16,7 +16,7 @@ int main(void)
 	$short s = 17;
 	$int i = -74874;
 	$bool b = 1;
-	$float f = 3.71;
+	$float f = (float) 3.71;
 	$long l = 487444;
 	$double dbl = 404.404;
 	$decimal dec;

src/interfaces/ecpg/test/expected/compat_informix-rnull.c

@@ -58,7 +58,7 @@ int main(void)
 
 	
 #line 19 "rnull.pgc"
- float f = 3.71 ;
+ float f = ( float ) 3.71 ;
 
 #line 19 "rnull.pgc"