simplify amalgamated header; remove duplicate extern 'C'
simplify amalgamated header; remove duplicate extern 'C'

StalderT/open62541,open62541/open62541,open62541/open62541,jpfr/open62541,JGrothoff/open62541,jpfr/open62541,StalderT/open62541,StalderT/open62541,open62541/open62541,JGrothoff/open62541,jpfr/open62541,jpfr/open62541,StalderT/open62541,JGrothoff/open62541,JGrothoff/open62541,open62541/open62541