package/open62541: fix build with nios2

package/open62541: fix build with nios2

Fix the following build failure with nios2 raised since bump to version
1.2.2 in commit	b436b82411305839abc76bb438fe92661a067526:

/home/buildroot/autobuild/instance-1/output-1/build/open62541-v1.2.2/src/ua_types_encoding_binary.c: In function 'unpack754':
/home/buildroot/autobuild/instance-1/output-1/build/open62541-v1.2.2/src/ua_types_encoding_binary.c:322:12: error: conversion to 'long double' from 'long long int' may alter its value [-Werror=conversion]
     result /= (1LL<<significandbits);
            ^~

Fixes:
 - http://autobuild.buildroot.org/results/c0a33dbada549c480bc741c1c8177b788ff053db

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>