librz/arch/alpha: build only for Capstone > 5

[XVilka]

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• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
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Detailed description

Fix Capstone 4 and 5 building:

FAILED: librz/arch/librz_arch.so.0.8.0.p/p_arch_alpha.c.o 
gcc -Ilibrz/arch/librz_arch.so.0.8.0.p -I. -I.. -Ilibrz -I../librz -Ilibrz/include -I../librz/include -I../librz/arch/isa -I../librz/arch/isa_gnu -Ilibrz/arch -I../librz/arch -Ilibrz/util/sdb/src -I../librz/util/sdb/src -I../librz/bin/format -I../librz/type/parser -I../subprojects/capstone-4.0.2/include -I../subprojects/capstone-4.0.2/include/capstone -Isubprojects/rzspp -I../subprojects/rzspp -fdiagnostics-color=always -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -Werror -O3 -DCC_SUPPORTS_W_ENUM_CONVERION -DCC_SUPPORTS_W_ENUM_COMPARE --std=gnu99 -Werror=sizeof-pointer-memaccess -Wimplicit-fallthrough=3 -fvisibility=hidden -DRZ_PLUGIN_INCORE=1 -DSUPPORTS_PCRE2_JIT -D_GNU_SOURCE -fPIC -MD -MQ librz/arch/librz_arch.so.0.8.0.p/p_arch_alpha.c.o -MF librz/arch/librz_arch.so.0.8.0.p/p_arch_alpha.c.o.d -o librz/arch/librz_arch.so.0.8.0.p/p_arch_alpha.c.o -c ../librz/arch/p/arch_alpha.c
In file included from ../librz/arch/p/arch_alpha.c:6:
../librz/arch/p/analysis/analysis_alpha_cs.c:10:10: fatal error: capstone/alpha.h: No such file or directory
   10 | #include <capstone/alpha.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

Test plan

CI is green

Closing issues

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