devel/boost-libs: fix on armv7

@progbits is spelled %progbits on armv7. This error caused dependent
port lang/emilua to fail to build, but does not directly affect this
port as the affected header files are not part of any translation unit
built with this package.

Approved by: portmgr (build fix blanket)
MFH: 2025Q1

(cherry picked from commit 355e55d2e4e5921b4aa98fe127849c7161b5867b)