Fix lang/luajit object file .strtab section size

When luajit writes a ELF object file (using the -b option), it sets the
size of the .strtab section one byte too short. In function
bcsave_elfobj(), the offset of the .rodata section after .strtab is
already calculated correctly, but the size of .strtab itself is one byte
too small. Even though there is a zero byte after the last string in
the table, the short size causes lld (the LLVM linker) to show an error
message similar to:

ld: error: obj/bytecode.o: string table non-null terminated

Fix it by increasing the size of the .strtab section by one byte. This
change has also been accepted upstream, but there is no new stable
release yet.

Approved by: portmgr (antoine)
PR: 223688