fix build regression on mips64 due to endian.h removal
commit 4d3a162d overlooked that the mips64 reloc.h dependent on endian.h not only for setting the ABI ldso name to match the byte order, but also for use of the byte swapping macros. they are needed to override R_TYPE, R_SYM, and R_INFO, to compensate for a mips "quirk" of always using big endian order for symbol references in relocations. part of that commit canot be reverted because the original code was wrong: it's invalid to define _GNU_SOURCE or any feature test macro in reloc.h, or anywhere except at the top of a source file. however, thanks to commit 316730cd, the feature test macro is no longer needed to access the endian-swapping macros, so simply bringing back the #include directive suffices.