Include bison header files into implementation files

Include bison header files into implementation files

Before Bison 3.4, the generated parser implementation files run afoul
of -Wmissing-variable-declarations (in spite of commit ab61c40bfa2)
because declarations for yylval and possibly yylloc are missing.  The
generated header files contain an extern declaration, but the
implementation files don't include the header files.  Since Bison 3.4,
the generated implementation files automatically include the generated
header files, so then it works.

To make this work with older Bison versions as well, include the
generated header file from the .y file.

(With older Bison versions, the generated implementation file contains
effectively a copy of the header file pasted in, so including the
header file is redundant.  But we know this works anyway because the
core grammar uses this arrangement already.)

Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org