fundamental: fix compile check for explicit_bzero

Our HAVE_* variables are defined to 0 or 1, so '#if defined(HAVE_*)' is always true.
The variable is not defined when compiling for EFI though, so we need the
additional guard.

Fixup for 3f92dc2fd4070b213e6bc85263a9bef06ec9a486.

(I don't want to do something like add -DHAVE_EXPLICIT_BZERO=0 to the commandline
in src/efi/boot/meson.build, because this quite verbose. Our compilation commandlines
are very long already. Let's instead keep this localized in this one spot in the
source file.)x

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