sim: sh: fix nested braces in struct init

The op struct includes an array of strings, but doesn't use braces
around that array when initializing.  This causes a ton of warnings
when using -Wmissing-braces.  Add them to fix.

The code this tool generates is the same before & after.

sim/configure

@@ -16561,7 +16561,6 @@ build_warnings="$build_warnings
 -Wno-enum-conversion
 "
 build_build_warnings="
--Wno-missing-braces
 -Wno-stringop-truncation
 -Wno-implicit-fallthrough
 -Wno-shadow=local

sim/m4/sim_ac_option_warnings.m4

@@ -77,8 +77,6 @@ dnl Disable until we can figure out how to make this work.
 -Wno-enum-conversion
 "
 build_build_warnings="
-dnl TODO Fix the sh/gencode.c which triggers a ton of these warnings.
--Wno-missing-braces
 dnl TODO Figure out the igen code that triggers warnings w/FORTIFY_SOURCE.
 -Wno-stringop-truncation
 dnl Fixing this requires ATTRIBUTE_FALLTHROUGH support at build time, but we