rts/RtsFlags.c: mention that -prof too enables support for +RTS -l

When we run a program with +RTS -l (to produce an eventlog), while that program was linked against an RTS flavour that doesn't support it, we get the following error message:

<executable name>: the flag -l requires the program to be built with -eventlog or -debug

However, according to https://gitlab.haskell.org/ghc/ghc/blob/master/mk/config.mk.in#L292, building with -prof is also enough to support producing event logs. And this is indeed confirmed by https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/main/Packages.hs#L1825 where we can see that both -debug and -prof can do the trick, in addition to -eventlog.

This patch simply additionally advertises -prof as a way to get an executable that can produce event logs.